Disabled FFmpeg decoder tests on Android in audio_decoder_unittests.cc
and included this file in the build on Android platform.

On Android, a hash value for one Opus decoded buffer is slightly off,
this CL also introduces and uses a hash comparison function that
accepts a tolerance.

BUG=542910

Review URL: https://codereview.chromium.org/1727953005

Cr-Commit-Position: refs/heads/master@{#378374}