[Xamarin.Android.Build.Tasks] Reduce rebuild cascades (#549)

Context: 518e57c

The scenario: rebuild `Xamarin.Android.Build.Tasks.csproj`:

	$ xbuild src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj

*If nothing has changed*, the expectation is that this should be
reasonably quick, because *nothing has changed*.

Unfortunately, that's not the case; a rebuild could take upwards of
30sec on my local machine, becaues of rebuild cascades:

	Target CoreCompile needs to be built as input file 'Xamarin.Android.Tools.BootstrapTasks/GenerateProfile.cs' is newer than output file 'obj/Debug/Xamarin.Android.Tools.BootstrapTasks.dll'
	Target _BuildJNIEnv needs to be built as input file '../../bin/BuildDebug/jnienv-gen.exe' is newer than output file 'Android.Runtime/JNIEnv.g.cs'
	Target CoreCompile needs to be built as input file 'Android.Runtime/JNIEnv.g.cs' is newer than output file 'obj/Debug/android-25/Mono.Android.dll'
	Target _GenerateMonoAndroidDex18 needs to be built as input file '../../bin/Debug/lib/xbuild-frameworks/MonoAndroid/v7.1/mono.android.jar' is newer than output file '../../bin/Debug/lib/xbuild-frameworks/MonoAndroid/v7.1/mono.android.dex'
	Target _CopyExtractedMultiDexJar needs to be built as input file '$HOME/android-toolchain/sdk/extras/android/m2repository/com/android/support/multidex/1.0.1/multidex-1.0.1.aar' is newer than output file '../../bin/Debug/lib/xbuild/Xamarin/Android/../../../mandroid/android-support-multidex.jar'

...and if I'm *really* unlucky:

	Target _BuildUnlessCached needs to be built as input file '.../xamarin-android/external/mono/autogen.sh' is newer than output file '../../bin/Debug//lib/xbuild-frameworks/MonoAndroid/v1.0/FSharp.Core.dll'

...as that means an `external/mono` rebuild (!).

Most of these are due to missing `<Touch/>` task use, to ensure that a
created/generated file is newer than the `Inputs` of the associated
target.

The `<GenerateProfile/>` task is slightly more complicated: in that
case, we only want the file timestamp to change if the file contents
have changed. Alter the `GenerateProfile.Execute()` logic to perform a
content diff before writing to the file, resulting in a new timestamp.

Finally, fix `GenerateJavaCallableWrappers` target use from
`Mono.Android.csproj` by providing a "real" value for
`$(JavaCallableWrapperAbsAssembly)` so that the
`GenerateJavaCallableWrappers` target's `Inputs` and `Outputs`
reference valid (existing) files. The previous value used a
`$(JavaCallableWrapperOutputPathAbs)` property, which doesn't appear
to have been defined anywhere, and thus was `""`.

Performing these changes reduces my typical "no change"
`Xamarin.Android.Build.Tasks.csproj` rebuild time from ~30sec down to
a more reasonable ~10sec, which is much better (though more than I'd
personally like).

Author: jonpryor

build-tools/mono-runtimes/mono-runtimes.targets

@@ -295,7 +295,7 @@
         WorkingDirectory="$(IntermediateOutputPath)\%(_MonoRuntime.Identity)"
     />
     <Touch
-        Files="@(_RuntimeSource);@(_ProfilerSource);@(_MonoPosixHelperSource);@(_BclProfileItems)"
+        Files="@(_RuntimeSource);@(_ProfilerSource);@(_MonoPosixHelperSource);@(_BclProfileItems);@(_MonoBtlsSource)"
     />
   </Target>
   <Target Name="_InstallRuntimes"
@@ -459,6 +459,9 @@
         SourceFiles="@(_FSharp)"
         DestinationFolder="$(_BclFrameworkDir)"
     />
+    <Touch
+        Files="@(_FSharp->'$(_BclFrameworkDir)\%(Filename)%(Extension)')"
+    />
     <Touch
         Files="@(_BclInstalledItem)"
     />

src/Mono.Android/Mono.Android.csproj

@@ -315,7 +315,7 @@
   </PropertyGroup>
   <Import Project="Mono.Android.targets" />
   <PropertyGroup>
-    <JavaCallableWrapperAbsAssembly>$(JavaCallableWrapperOutputPathAbs)$(AssemblyName).dll</JavaCallableWrapperAbsAssembly>
+    <JavaCallableWrapperAbsAssembly>$([System.IO.Path]::GetFullPath ('$(OutputPath)$(AssemblyName).dll'))</JavaCallableWrapperAbsAssembly>
     <JavaCallableWrapperAfterTargets>CoreBuild</JavaCallableWrapperAfterTargets>
   </PropertyGroup>
   <Import Project="..\..\build-tools\scripts\JavaCallableWrappers.targets" />

src/Mono.Android/Mono.Android.targets

@@ -40,6 +40,7 @@
     <Exec
         Command="$(ManagedRuntime) ..\..\bin\Build$(Configuration)\jnienv-gen.exe -o Android.Runtime\JNIEnv.g.cs --use-java-interop"
     />
+    <Touch Files="Android.Runtime\JNIEnv.g.cs" />
   </Target>
   <ItemGroup>
     <_AndroidProfile Include="Profiles\api-*.xml.in" />
@@ -128,6 +129,7 @@
         SourceFiles="$(IntermediateOutputPath)__dex\classes.dex"
         DestinationFiles="$(OutputPath)mono.android.dex"
     />
+    <Touch Files="$(OutputPath)mono.android.dex" />
     <RemoveDir
         Directories="$(IntermediateOutputPath)__dex"
     />

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.targets

@@ -143,9 +143,11 @@
     <Copy
       SourceFiles="$(IntermediateOutputPath)multidex-aar\classes.jar"
       DestinationFiles="$(OutputPath)..\..\..\mandroid\android-support-multidex.jar" />
+    <Touch Files="$(OutputPath)..\..\..\mandroid\android-support-multidex.jar" />
     <Copy
       SourceFiles="$(_SupportLicense)"
       DestinationFiles="$(OutputPath)..\..\..\mandroid\MULTIDEX_JAR_LICENSE" />
+    <Touch Files="$(OutputPath)..\..\..\mandroid\MULTIDEX_JAR_LICENSE" />
   </Target>
 
   <ItemGroup>

src/Xamarin.Android.Tools.BootstrapTasks/Xamarin.Android.Tools.BootstrapTasks/GenerateProfile.cs

@@ -38,7 +38,14 @@ public override bool Execute ()
 			sb.AppendLine ("\t}");
 			sb.AppendLine ("}");
 
-			File.WriteAllText (OutputFile.ItemSpec, sb.ToString ());
+			var newContents = sb.ToString ();
+			var curContents = "";
+			if (File.Exists (OutputFile.ItemSpec)) {
+				curContents = File.ReadAllText (OutputFile.ItemSpec);
+			}
+			if (newContents != curContents) {
+				File.WriteAllText (OutputFile.ItemSpec, sb.ToString ());
+			}
 
 			return !Log.HasLoggedErrors;
 		}