policy_hal: Override stopOutput method in policy_hal

-Ringtone is not heard over BT and speaker once MT call is
 disconnected. When the call is disconnected from
 MO device, only one output is getting unmuted as
 part of stopSource of base AudioPolicy.

-stopSource is called from stopOutput, as stopOutput
 is not overriden in CustomPolicy, stopSource of base class
 is getting called and only one output is getting unmuted

-Overriding stopOutput in CustomPolicy, so that stopSource of
 CustomPolicy gets called and all the outputs are unmuted in
 case of duplicate outputs.

CR's fixed: 897000

Change-Id: I8c0a99dbaae860a261ce2f15b6056d3d9c545568