[Xamarin.Android.Build.Tasks] cache java -version output

[jonathanpeppers]

Context: https://github.com/Microsoft/msbuild/blob/f147a76a60bf9e516800f65f9614c914df0a4f15/src/Framework/IBuildEngine4.cs#L34-L84
Context: https://github.com/xamarin/Xamarin.Forms/blob/42c07d1ae5aa56eb574b7d169499f1a9af7ec080/Xamarin.Forms.ControlGallery.Android/Xamarin.Forms.ControlGallery.Android.csproj

The new ValidateJavaVersion MSBuild task gets invoked for every
project on every build, regardless if there are changes or not.
Unfortunately, this task also takes around 250ms to run, since it
shells out to java -version and javac -version.

We can use MSBuild's GetRegisteredTaskObject and
RegisterTaskObject to cache the output of running these command line
tools. If we key with the full path to java or javac, the cache
should be properly invalidated if a different JavaSdkPath is passed
in. I added a new set of ValidateJavaVersionTests to directly test
this caching functionality, plus a couple smoke tests.

In the case of Xamarin.Forms.ControlGallery.Android:

• ValidateJavaVersion runs 7 times
• A build (with no changes) went from 12.820s to 11.185s, saving
  ~1.635s of build time
• I also tested the changes in VS Windows, and was able to see logs
  indicating the value is cached.

Improvements to MockBuildEngine:

• General code formatting / refactoring
• Added a Messages list that can be asserted against
• Refactored the Task APIs so that they are fully functional

Other changes:

• Since BaseTest now has a SetUp class, we can't have a method
  named SetUp or we get a warning. I used a Setup method (with
  lowercase u) instead, and modified KeyToolTests to fix this
  warning.

[jonathanpeppers]

Let's wait and make sure #1916 is good (bumped in monodroid) before we merge this one.

[jonathanpeppers]

#1916 is now in monodroid/master, things seem to be OK.