Reset system audio mode on startup rather than on stop.

Bug fixed - When TV was turned off and AVR was on, audio was routed back
to TV momentarily before the TV turned off.

b/19791264

Change-Id: Iad639b6ba151b6615a7f4ce49fddfa4fc485e16c
(cherry picked from commit b0efbf3898e80181210103f6ac535af69d3abb2d)