AAPT2: Change proto format to reduce usage of StringPool

The StringPool class is a binary blob and makes it difficult to
modify the proto files from external tools (like bundle-tool).

Size increase of full build is negligible.

Test: make aapt2_tests
Bug: 65645766
Change-Id: I984755170c315730ab751b51133f8fb2f614f6af
Merged-In: I984755170c315730ab751b51133f8fb2f614f6af