[build] Use platform-tools r31

Context: dotnet/java-interop#897
Context: f6011d9

Commit f6011d9 updated external/xamarin-android-tools so that the
Android SDK platform-tools r31.0.3 package would be preferred, but
*didn't* update `$(XAPlatformToolsVersion)` to cause the
xamarin-android build to use platform-tools r31.0.3, because:

> The reason to *not* bump `$(XAPlatformToolsVersion)` is that
> attempting to do so breaks the `src/Mono.Android` build:
>
>  1. platform-tools r31.0.3 doesn't contain
>     `platform-tools/api/annotations.zip`, which is used by
>     `generator` to emit certain custom attributes such as
>     `RequiresPermission`.
>
>  2. While (1) can be worked around by instead using
>     `$(AndroidSdkDirectory)/platforms/android-*/data/annotations.zip`,
>     this introduces API changes reported by the
>     `_CheckApiCompatibility` target, in particular changes due to
>     custom attribute string changes.  These string changes happen
>     because Google ships *invalid XML* in `data/annotations.zip` for
>     API-29+ (?!):

dotnet/java-interop#897 is a proposed fix for (2), and should allow
us to use platform-tools r31.0.3 for the xamarin-android build.

WIP WIP WIP WIP

Context: dotnet/java-interop#897 (comment)

This does not fully build for me locally, because "duplicate" custom
attributes are being emitted:

	$ diff -u obj/Debug/monoandroid10/android-{27,28}/mcw/Android.App.WallpaperManager.cs
	@@ -553,7 +553,7 @@
	                // Metadata.xml XPath method reference: path="/api/package[@name='android.app']/class[@name='WallpaperManager']/method[@name='clear' and count(parameter)=1 and parameter[1][@type='int']]"
	                [global::System.Runtime.Versioning.SupportedOSPlatformAttribute ("android24.0")]
	                [Register ("clear", "(I)V", "GetClear_IHandler", ApiSince = 24)]
	-               [global::Android.Runtime.RequiresPermission ("android.permission.SET_WALLPAPER")]
	+               [global::Android.Runtime.RequiresPermission ("android.permission.SET_WALLPAPER")][global::Android.Runtime.RequiresPermission ("android.permission.SET_WALLPAPER")]
	                public virtual unsafe void Clear ([global::Android.Runtime.GeneratedEnum] Android.App.WallpaperManagerFlags which)
	                {
	                        const string __id = "clear.(I)V";

resulting in errors such as:

	CannotChangeAttribute : Attribute 'Android.Runtime.RequiresPermissionAttribute' on 'Android.App.WallpaperManager.Clear(Android.App.WallpaperManagerFlags)' changed from '[RequiresPermissionAttribute("android.permission.SET_WALLPAPER")]' in the contract to '[RequiresPermissionAttribute("android.permission.SET_WALLPAPER")]' in the implementation.

We should similarly fixup `generator` so that these duplicate
custom attributes are *not* emitted.

Author: jonpryor

src/Mono.Android/Mono.Android.targets

@@ -135,11 +135,15 @@
   </Target>
   <Target Name="_GenerateBinding"
       BeforeTargets="CoreCompile"
-      Inputs="metadata;enumflags;map.csv;methodmap.csv;$(IntermediateOutputPath)mcw\api.xml"
+      Inputs="$(MSBuildThisFileFullPath);metadata;enumflags;map.csv;methodmap.csv;$(IntermediateOutputPath)mcw\api.xml"
       Outputs="$(IntermediateOutputPath)mcw\Mono.Android.projitems">
     <MakeDir Directories="$(IntermediateOutputPath)mcw" />
     <PropertyGroup>
       <_PlatformIdVersions>$(AndroidSdkDirectory)\platforms\android-$(AndroidPlatformId)\data\api-versions.xml</_PlatformIdVersions>
+      <_AndroidPlatformAnnotations>$(AndroidSdkDirectory)/platforms/android-$(AndroidPlatformId)/data/annotations.zip</_AndroidPlatformAnnotations>
+      <_PlatformToolsAnnotations>$(AndroidSdkDirectory)/platform-tools/api/annotations.zip</_PlatformToolsAnnotations>
+      <_AnnotationsZip Condition=" Exists($(_AndroidPlatformAnnotations)) ">$(_AndroidPlatformAnnotation)</_AnnotationsZip>
+      <_AnnotationsZip Condition=" '$(_AnnotationsZip)' == '' And Exists('$(_PlatformToolsAnnotations)') ">$(_PlatformToolsAnnotations)</_AnnotationsZip>
     </PropertyGroup>
     <ItemGroup>
       <_ApiVersion
@@ -160,7 +164,7 @@
       <_Fixup>--fixup=metadata</_Fixup>
       <_Enums1>--preserve-enums --enumflags=enumflags --enumfields=map.csv --enummethods=methodmap.csv</_Enums1>
       <_Enums2>--enummetadata=$(IntermediateOutputPath)mcw\enummetadata</_Enums2>
-      <_Annotations>--annotations="$(AndroidSdkDirectory)/platform-tools/api/annotations.zip"</_Annotations>
+      <_Annotations Condition=" '$(_AnnotationsZip)' != '' ">"--annotations=$(_AnnotationsZip)"</_Annotations>
       <_Assembly>--assembly="Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null"</_Assembly>
       <_TypeMap>--type-map-report=$(IntermediateOutputPath)mcw\type-mapping.txt</_TypeMap>
       <_Api>$(IntermediateOutputPath)mcw\api.xml</_Api>