Only remove overlay reference once it is faded out

This allows to remove the overlay in case of exit transition that
comes before the overlay is faded out - in such cases the overlay
handling code would skip removal due to PipTransitionState.UNDEFINED
state set after the exit transition.

Bug: 312050085
Test: before http://recall/-/ekEuGtt9d9HWqkUtAzpHx8/cY9l8Yv2NIB1Bo30xGfnWn
Test: after http://recall/-/ekEuGtt9d9HWqkUtAzpHx8/bIau9G9vKCmm1Q9liuS5q5
Test: note that videos are taken with the fadeout delay set to 2000 ms
for easier visualization
Flag: none

Change-Id: I59fea21fd640719d507619dd7c34ca3c5445c760