[XABT] Make assembly compression incremental.

[jpobst]

If using $(AndroidEnableAssemblyCompression) today, we do not compress assemblies incrementally. That is, we always compress every assembly even if they haven't changed since the previous build.

Update the CompressAssemblies task to check if the input is newer than the output, and skip recompressing if the input has not changed.

This is most visible with the following test case:

dotnet new android
dotnet build -p:EmbedAssembliesIntoApk=true -p:AndroidIncludeDebugSymbols=false

Scenario (CompressAssemblies tasks)	main	This PR
Full	44.26 s	44.2 s
NoChanges	not run	34 ms
ChangeResource	27.8 s	9 ms
AddResource	25.81 s	25 ms
ChangeCSharp	25.87 s	30 ms
ChangeCSharpJLO	27.04 s	23.36 s

ChangeCSharpJLO should be similarly reduced, however something earlier in the build process is causing Mono.Android.dll to get touched so it is getting recompressed. We'll leave that as an investigation for another day.

[dellis1972]

Reviewing the full code file this does not seem to be enabled anymore? The code which raised this issue is commented out.

[jpobst]

I don't know why that error condition is disabled in the existing code, but I think it's fine to ensure we log it properly if it ever gets reenabled.

[jonathanpeppers]

Could this test be updated:

android/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/IncrementalBuildTest.cs

Lines 37 to 40 in 18af3ac

	Assert.IsTrue (b.Build (proj), "third build failed");
	Assert.IsFalse (
	b.Output.IsTargetSkipped ("_Sign"),
	"the _Sign target should run");

To check:

b.Output.AssertTargetIsPartiallyBuilt ("_CompressAssemblies"); 

[jonathanpeppers]

You might also add:

Suggested change

	Inputs="@(_AssembliesToCompress);@(_AndroidMSBuildAllProjects)"
	Inputs="@(_AssembliesToCompress);@(_AndroidMSBuildAllProjects);$(_AndroidBuildPropertiesCache)"

The build.props file has been the thing used to rerun all targets no matter what.

[jpobst]

Updated with feedback, I think this is ready.