Empty output if compact fails.

There must be equal number of start/end calls with right token given in
order, if used in wrong order, ProtoOutputStream will have empty output.

Also refactor Android.bp, so the gtest is standalone unit test, it used
to require compile/push libprotoutil.so to device.

Bug: 77342154
Test: atest libprotoutil_test
Change-Id: I0011bbab34c04cb38164d2ed21cd818d52a2ecf9