Always cancel insets animation if control target is gone

Otherwise if updateControlForTarget is called when consuming
mHasPendingPosition with mPendingControlTarget==null in
mSetControlPositionConsumer, there could be NPE that mControl
is set to null from notifyControlTargetChanged.

Besides, it is unnecessary to handle position by
mSetControlPositionConsumer if the control target is gone.
So simply cancel the animation rather than adding null check.

Fix: 433587055
Flag: EXEMPT bugfix
Test: InsetsSourceProviderTest#testUpdateControlForTarget
Change-Id: Ia8e34bc583278c691f053ea07f375a451854e789