Add Support for AndroidManifest.xml overlays

[dellis1972]

Users would like the ability to merge AndroidManifest.xml files with additional debugging settings etc.
See #3428 (comment)

It looks like I had the foresight to add a ManifestOverlayFiles property to the ManifestMerger Task, but neglected to hook it up in the targets. This is the exact property users would need.

Adding a new build action like AndroidManifestOverlay might do the trick. This can then be passed in to the manifest merger as an overlay on top of the other files.

Then Users might be able to do something like this.