Automatically export generated .pb.h files.

- Automatically export the include path of the generated .pb.h files
- Set up the dependency to make sure the .pb.h files get generated
  before any dependent c/c++ files get compiled.
With this change, any module that links a proto library doesn't need
any extra setup.

Bug: 14563418
Change-Id: Iea7d19e9d8dce8e7d479c386b7a6151a95a0a0df