[ Merge of http://go/wvgerrit/119804 ]
The coverage framework used by Android requires the process to call
exit() for coverage information to be dumped after the tests.
Android unit tests have been adapted to use the a new main module.
The core test_main.cpp used by integration tests has been updated to
call exit().
Bug: 138941105
Test: Linux and Android unittests
Change-Id: Ifffc7b8290c50dffe527738a36547c1d2fb90bd3