Extract the mapping of flags to APIs.

This CL creates a binary that extracts flagged APIs from an API
signature file, and writes the mapping of flags to APIs to a pb file.

Bug: 311059624
Test: mmm -j frameworks/base/api/coverage/tools/:extract-flagged-apis
      extract-flagged-apis <input api.text file> <output pb file>
Change-Id: I9b6f45bd126cd6a5b2ed1e9f83bdaa12dd580480