Added IncrementalFileMerger; update MergeJavaRes

Added IncrementalFileManager and associated classes. This
class is capable of doing a perfect incremental merge of
sets of relative files into a destination file, keeping
track of where information came from. It makes java res
merging incremental.

Test: Included
Change-Id: Iddc28655bb00e7859ebd9b5dc04093f6d886ea05