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This change cleans the packages' intermediate files if their overlays
changed between incremental builds.
If two builds have different overlay, they will have different R
classes, and so the jar files in the common intermediate dirs can
not be shared. Therefore incremental build can't be applied.

This change detects the overlay changes on package's base.
If a package's overlay is different from the previous build, its common
intermediate dir is nuked.
This makes broader incremental builds possible.

Change-Id: I368610ebbbbc85a80f9aecd714ab22cd78da7f12