libdebuggerd: add protobuf implementation.

This commit implements protobuf output for tombstones, along with a
translator that should emit bytewise identical output to the existing
tombstone dumping code, except for ancillary data from GWP-ASan and
Scudo, which haven't been implemented yet.

Test: setprop debug.debuggerd.translate.translate_proto_to_text 1 &&
        /data/nativetest64/debuggerd_test/debuggerd_test
Test: for TOMBSTONE in /data/tombstones/tombstone_??; do
        pbtombstone $TOMBSTONE.pb | diff $TOMBSTONE -
      done
Change-Id: Ieeece6e6d1c26eb608b00ec24e2e725e161c8c92