This change introduces idmap parsing of <overlay> tags.
The <overlay> tag allows one to explicitly map resources in the target
to either a resource in the overlay or an inline attribute value.
Use the android:resourcesMap atttribute on the <overlay> tag in the
android manifest to specify a file to provide the resource mapping.
Bug: 135943783
Bug: 135051420
Test: idmap2_tests
Change-Id: I1740dcdc01849c43b1f2cb8c6645d666dbb05dba
9e4f52b2e5