Add tests to verify volume command behavior

Tests verify that a Playback device will send volume commands to the
correct target device. Verify that volume commands can be sent both when
the device is active source and not the active source.

Test: atest HdmiCecLocalDevicePlaybackTest
Bug: 161052729
Change-Id: I592451412203efb53f0d8a9dbbe7f8c80668df77