Add encoding of name and actor overlayable fields

Encode the actor and name attributes of <overlayable> elements in the
overlayable chunks. Two chunks cannot have the same name.

Bug: 110869880
Bug: 119390855
Test: aapt2_tests and libandroidfw_tests
Change-Id: I0ed95efef640b3927046b6155d1840f0d96030dd