Add setTriggerBack interface to BackCallback.

This allows the back arrow to report back trigger state to BackAnimation
as the gesture is going on. The trigger state might be included in the
future back progress API.

Bug: b/238475284
Test: Manual for now, because the current back arrow doesn't have tests
unfortunately, and will be deprecated soon. We should add tests when we productionize the
arrow prototype.

Change-Id: Idcbc2244d942788cd18a07ece252b0e7b6ef849a