Allow apps receiving media button events to start FGS from BG.

Bug: 174699413
Test: 1. Verified that media button receiver can start FGS when it is
      started due to user pressing play/pause button on headset.
      2. Verified the same after rebooting the device.
Change-Id: If6ccaa7af68aa3f3d1034db33e9fbafff9eb7669