Issue 3395734: fix media server restart detection

Commit 25101b0b in AudioSystem for issue 3395734 broke the mechanism
in AudioService to poll and detect the restarting of media server
process after a crash.

The fix consists in changing the method used to poll AudioSystem and
trigger a callback indicating success when media server is responding
(the method must be on IAudioFlinger and not IAudioService to trigger the callback).

Change-Id: I843c0194de9cbf82c779cabbf2f3cbf8064603c8