AAPT2: Switch to protobuf for intermediate format

Without needing to conform to the runtime data format,
it is much easier to add new features such as debugging symbols
and carrying over product data to link time.

This also simplifies the runtime format parser and serializer,
which will change much less frequently than the protobuf intermediate
format.

Change-Id: I209787bbf087db0a58a534cb8511c51d21133e00