bootanimation: stop the active output for boot sound before exit

-when bootanimatoin process exited and the output used for boot
 sound is still active, it could trigger an infinite loop in
 AudioPolicyManager and cause subsequent ANRs
-ensure the active output to be stopped before bootanimation's exit

Change-Id: I2f7e17a403d606442dcc9d87acadedac23c2a171
CRs-Fixed: 759219