[Xamarin.Android.Build.Tasks] fix warning when missing proguard-android.txt (#8641)

We are seeing numerous test failures because of this warning.

    warning XA4304: ProGuard configuration file 'C:\Android\android-sdk\tools\proguard\proguard-android.txt' was not found.

This is because the `proguard-android.txt` file no longer ships with the
Android SDK. We should check this file exists before adding it to the
list of `@(_ProguardConfiguration)` files. This way if a user still does
have it, it will be included.

Author: dellis1972

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

@@ -1940,7 +1940,7 @@ because xbuild doesn't support framework reference assemblies.
     <_ProguardConfiguration Include="$(ProguardConfigFiles)" />
   </ItemGroup>
   <ItemGroup Condition=" '$(ProguardConfigFiles)' == '' ">
-    <_ProguardConfiguration Include="$(_AndroidSdkDirectory)tools\proguard\proguard-android.txt" />
+    <_ProguardConfiguration Include="$(_AndroidSdkDirectory)tools\proguard\proguard-android.txt"     Condition=" Exists ('$(_AndroidSdkDirectory)tools\proguard\proguard-android.txt') " />
     <_ProguardConfiguration Include="$(IntermediateOutputPath)proguard\proguard_xamarin.cfg"         Condition=" '$(AndroidLinkTool)' != '' " />
     <_ProguardConfiguration Include="$(_ProguardProjectConfiguration)"                               Condition=" '$(AndroidLinkTool)' != '' " />
     <_ProguardConfiguration Include="$(IntermediateOutputPath)proguard\proguard_project_primary.cfg" Condition=" '$(AndroidLinkTool)' != '' " />