[build] set file extension for common ToolExe values (#8267)

Running the `dotnet-local.cmd -t:Install` target with a local
xamarin-android build on Windows, I got the error:

	AndroidAdb
	Errors
	    Xamarin.Android.Common.targets(2641,3): error MSB6004:
	    The specified task executable location "D:\android-toolchain\sdk\platform-tools\adb" is invalid.

This appears to only happen with a local of xamarin/xamarin-android
because it specifies `$(AdbToolExe)`=adb in `Configuration.props`.

It appears that most of our tasks just use the pattern:

	partial class ToolTaskSubclass : /* eventually… */ ToolTask {
	    protected override string GenerateFullPathToTool ()
	    {
	        return Path.Combine (ToolPath, ToolExe);
	    }
	}

Where if `ToolExe` was completely blank, it would fall back to:

	partial class ToolTaskSubclass {
	    protected override string ToolName => OS.IsWindows ? "adb.exe" : "adb";
	}

Searching the web:

  * <https://grep.app/search?current=2&q=ToolExe&words=true&filter[lang][0]=C%23&filter[lang][1]=XML>

It seems most other tasks use `ToolExe` as-is, if it is passed in.
Where most of the time the default value is blank.

I think the simple fix here is to specify the file extension in
`Configuration.props` for any of our `$(*ToolExe)` values.

I also removed `$(AndroidToolExe)` because it is unused and may not
event exist anymore in the Android SDK.

Author: jonathanpeppers

Configuration.props

@@ -180,20 +180,22 @@
   </PropertyGroup>
   <PropertyGroup>
     <AdbToolPath Condition=" '$(AdbToolPath)' == '' ">$(AndroidSdkFullPath)\platform-tools\</AdbToolPath>
-    <AdbToolExe Condition=" '$(AdbToolExe)' == '' ">adb</AdbToolExe>
+    <AdbToolExe Condition=" '$(AdbToolExe)' == '' and '$(HostOS)' != 'Windows' ">adb</AdbToolExe>
+    <AdbToolExe Condition=" '$(AdbToolExe)' == '' and '$(HostOS)' == 'Windows' ">adb.exe</AdbToolExe>
     <AvdManagerHome Condition=" '$(AvdManagerHome)' == '' ">$(AndroidToolchainDirectory)</AvdManagerHome>
-    <AvdManagerToolExe Condition=" '$(AvdManagerToolExe)' == '' ">avdmanager</AvdManagerToolExe>
+    <AvdManagerToolExe Condition=" '$(AvdManagerToolExe)' == '' and '$(HostOS)' != 'Windows' ">avdmanager</AvdManagerToolExe>
+    <AvdManagerToolExe Condition=" '$(AvdManagerToolExe)' == '' and '$(HostOS)' == 'Windows' ">avdmanager.bat</AvdManagerToolExe>
     <AndroidToolPath Condition=" '$(AndroidToolPath)' == '' ">$(AndroidSdkFullPath)\tools</AndroidToolPath>
     <AndroidToolsBinPath Condition=" '$(AndroidToolsBinPath)' == '' ">$(AndroidToolPath)\bin</AndroidToolsBinPath>
-    <AndroidToolExe Condition=" '$(AndroidToolExe)' == '' ">android</AndroidToolExe>
     <CommandLineToolsFolder Condition=" '$(CommandLineToolsFolder)' == '' ">7.0</CommandLineToolsFolder>
     <CommandLineToolsVersion Condition=" '$(CommandLineToolsVersion)' == '' ">8512546_latest</CommandLineToolsVersion>
     <CommandLineToolsBinPath Condition=" '$(CommandLineToolsBinPath)' == '' ">$(AndroidSdkFullPath)\cmdline-tools\$(CommandLineToolsFolder)\bin</CommandLineToolsBinPath>
     <!-- Version numbers and PkgVersion are found in https://dl-ssl.google.com/android/repository/repository2-3.xml -->
     <EmulatorVersion Condition=" '$(EmulatorVersion)' == '' ">9364964</EmulatorVersion>
     <EmulatorPkgRevision Condition=" '$(EmulatorPkgRevision)' == '' ">32.1.9</EmulatorPkgRevision>
     <EmulatorToolPath Condition=" '$(EmulatorToolPath)' == '' ">$(AndroidSdkFullPath)\emulator</EmulatorToolPath>
-    <EmulatorToolExe Condition=" '$(EmulatorToolExe)' == '' ">emulator</EmulatorToolExe>
+    <EmulatorToolExe Condition=" '$(EmulatorToolExe)' == '' and '$(HostOS)' != 'Windows'  ">emulator</EmulatorToolExe>
+    <EmulatorToolExe Condition=" '$(EmulatorToolExe)' == '' and '$(HostOS)' == 'Windows'  ">emulator.exe</EmulatorToolExe>
     <NdkBuildPath Condition=" '$(NdkBuildPath)' == '' And '$(HostOS)' != 'Windows' ">$(AndroidNdkDirectory)\ndk-build</NdkBuildPath>
     <NdkBuildPath Condition=" '$(NdkBuildPath)' == '' And '$(HostOS)' == 'Windows' ">$(AndroidNdkDirectory)\ndk-build.cmd</NdkBuildPath>
     <BundleToolJarPath Condition=" '$(BundleToolJarPath)' == '' ">$(MicrosoftAndroidSdkOutDir)bundletool.jar</BundleToolJarPath>