AAPT Container Format (.apc) is a simple container that
enumerates the various intermediate files that AAPT2 generates
during the compile phase.
The format is defined in formats.md.
For now, continue using the .flat extension for the container file,
and keep making use of the .flata zip for storing multiple files.
This will allow easier integration with existing build systems and allow
the evolution of the APC format to better handle arbitrarily large
files.
Test: make aapt2_tests
Change-Id: Id7216e5b76316bdd683f0fa4eaf2d2da273ba815
