Initialization of audio manager for Android is now done on the audio thread.

Goal is to move heavy initialization tasks from the IO thread to the audio thread instead.

This CL also ensures that the Android audio unit test calls audio manager methods on the audio thread to ensure the same behavior as in Chrome. In addition, all input/output-streams will be created and used on the audio thread.
     
BUG=337867
TEST=Manual getSources demos, media_unittests, content_unittests and WebRTC apprtc demo

Review URL: https://codereview.chromium.org/131503006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252614 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252614