Abstract some transition checks so they can be expanded.

This is mostly to clean-up CL diffs so its easier to tell
what is changing. The main change here is replacing naked
parint == null checks with TransitionInfo.isIndependent() which
can do extra logic to handle cases where, even though a change
has a parent, it might be animating independently with in it.
The easiest example is display rotating while an app is opening.

This also fixes a small bug when removing a non-visible task.

Bug: 179270750
Test: atest TransitionTests
Change-Id: Ibd72b0721f33602b26bcc5c7060fc959aed04377