_UpdateAndroidResgen constantly being called

[dellis1972]

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=58169

We have a bit of a problem with the way we deal with embedding
resources (i.e AndroidResource) into an Android dll.
The process usually goes as follows

1. Collect all the resources into an intermediate directory
   %(IntermediateOutputPath)__libray_project_imports__
2. Create a zip file which contains all of those files
3. Embed that zip into the assembly when we compile.

This seems logical until you look at it from a repeat build
point of view. The senario is that a user changes some C# code
in the android dll. The build process currently does steps
1,2 for ALL changes. So we create a new zip and add that to
the dll. The result of which is as far as the application build
process is concerned its a new zip.. with new files. So we end up
running _AddLibraryProjectsEmbeddedResourceToProject and then
in turn _UpdateAndroidResgen which then in turn will probably
result in an apk build!

All of that happens even though we NEVER touched the resources, we only
touched the C# code.

So in order to fix this we need to look at things slightly differently.
Instead of creating a new zip each time we build, we should incrementally
update/refresh the one we have in the intermediate directory already.
This means not only just updating files but also REMOVING deleted files
from the existing zip file.

By doing this we can maintain the ModifiedDate values in the zip, which
can be used later in the build process to decide if thing were changed or
not.

So to implement this we need to change a few things. Firstly the task
RemoveUnknownFiles has been updated to output a new RemovedFiles
property. This is a ITaskItem which contains a list of the files that
were removed from the build. Next up CreateManagedLibraryResourceArchive
is modified to make use of this new data via the RemovedAndroidResourceFiles
property. We use this to delete files from the intermediate directory and
zip file which have been removed from the project.

We also update CreateManagedLibraryResourceArchive to make use of the
existing zip rather than creating a new one each time. As well as updating
the Files.ExtractAll to allow it to refresh files and remove item from the
zip which we do not expect.

The result of these changes means that changing C# code will result in the
existing zip from the previous build being used for resources. If we do
change a resource then only that item in the zip is refreshed.

These changes then filter down to the ResolveLibraryProjectImports task
which is responsible for maintaining the __library_projects__ directory.
We update this task to make use of the new information available to us in
the zip file to ensure that we only update files which have been changed.
We also only update the .stamp file if we have changed files.

The last piece of the puzzle is to change the _UpdateAndroidResgen target
to NOT use the @(ReferencePath);@(ReferenceDependencyPaths) ItemGroups
as its inputs. If we leave this as is even if we update the C# code the target
will still run because the dll is newer than the $(_AndroidResgenFlagFile).
So we alter this to use the various stamp files we produce to show the
resources were updated. This means that this task will run only when we have
a change.

A new unit test has been added to test out this entire process and to ensure that
the targets and things that should be updated and removed are.

[jonpryor]

I suspect that merging PR #700 caused the merge conflicts here. :-(

[dellis1972]

@jonpryor I knew that would happen :)

[dellis1972]

build

[dellis1972]

build