Define a unified protobuf message for "aapt2 optimize" metadata.

This helps solve three problems:

* obfuscation maps are now documented.
* obfuscation maps will not require hand-rolled parsers.
* no additional plumbing will be required for future
  optimizations---plumbing individual mapping files in Bazel for
  ProGuard, aapt2, etc. has been a pain.

Bug: 153201806
Test: make aapt2
Change-Id: Ie3829834527e8875d768ae28242d3aeca28c6db2