hal: audio_extn: fix pop noise when FM is interrupted by voice call

Minor pop is observed when FM playback is stopped, this is due to
derouting of loopback devices while they still have audible data.
This is causing pops even when FM is stopped after playback, but
this pop is more perceivable in case of interruption by voice call.
Fix the issue by draining unmuted data before derouting FM playback.

Change-Id: If92e43f3a15f0a9cfdf2819827c7f7a0ba852e3d