Ignore callbacks from the non-active hardware composer

Ignore callbacks from the non-active hardware composer so we don't get
e.g. duplicate vsync callbacks, one from each composer.

Bug: None

Test: Manually confirmed with logs we're now ignoring callbacks on the
      non-active composer.

Change-Id: I8b475d6283d86c64ff96b41e78528bce8c6ff1d3