AAPT2: Define intermediate compiled XML proto

This proto format is meant to encapsulate more information
that is specific to Android and allows for easier validation
and manipulation across tools.

Test: make aapt2_tests
Bug: 65645766
Change-Id: I13bc34a460671fc0a36246be0d287a3d37d244d6
Merged-In: I13bc34a460671fc0a36246be0d287a3d37d244d6