audioflinger: fix mmap stream volume

Fix bad casting from MmapPlaybackThread to VolumeInterface causing
volume commands to be lost by audio flinger for MMAP threads.

Bug: 37518243
Test: Verify volume is received by the audio HAL on MMAP streams
      Verify no regression for other stream types.

Change-Id: I584ae868245358b2edefecc46a5249092f8be229