Add unit tests and cleanup PipBoundsState

1. Adds missing unit tests
2. Cleans up public API (nullability and doc)
3. Renames AnimatingBoundsState to MotionBoundsState to improve
readability of what that state actually holds, that is, the state
of the "bounds in motion" which can be the bounds used during an
animation but also the bounds used during a drag or resize
operation.

Bug: 169373982
Test: atest com.android.wm.shell.pip
Change-Id: I951fa3ea1310b69c93b53a1690c0a1cf0632dd33