audio: Retained VOIP Mute State during device switch

On L, Audio Policy Manager closes all input streams
during device switch. Previous mute state is lost
when there is a device switch during VOIP call

Fix is to reset mute state only when the call is
disconnected and retain it during the device switches

Change-Id: Iaa2ad525db92fdad4d598680f5ba96e74a8c3b9c
CRs-Fixed: 773035