Check the fullscreen magnification state onTransactionCommitted

NullPointerException could happen when (1) create fullscreen
magnification border (2) remove fullscreen magnification (3) transaction
for creating border is committed and try to run animation on the border
view that has become null. Therefore, we should check the state before
performing animation.

Bug: 395005528
Test: atest FullscreenMagnificationControllerTest
Flag: EXEMPT BUGFIX
Change-Id: I7e7eff65db8cae583533a41b71965440d6f348f7