schedule a PIP update even when the previous stack is the current stack.

From commit 6db8fb27, we now only create
a new stack when there is more than one activity inside the PIP stack.
Additionally, when we expand (exit PIP), we no longer reparent to a
fullscreen stack - we simply do a windowing mode change on the existing
stack.

For expanding, the behavior is now that prevStack == currentStack, so
removing this condition.
For entering PIP, it is still possible that prevStack != currentStack
(multi. activity case) - in that case, we need to ensure one of the new
stack is in pinned mode.

Bug: 144824244
Test: Open Play Movies in portrait mode, play any movie - hit home to
enter PIP. Expand PIP. No longer experience black screen; see from logs
that scheduleUpdatePictureInPictureIfNeeded is called.

Change-Id: I3232d74110618919fcacbc9edd4eb3cb02f55015