Don't skip cleaning up PIP state in onTaskVanished even if EXITING_PIP

onTaskVanished's clean up work is sometimes skipped when the current
state of PIP is EXITING_PIP because that is considered to be a "not
in pip state". Instead, make sure clean up does happen and only skip
it if the state is already UNDEFINED (i.e. if onTaskVanished is called
twice or before onTaskAppeared).

Bug: 176302696
Test: Enter PIP with maps, go back to fullscreen, make sure back gesture
works

Change-Id: I81e862f1d24892e565d1f8fcf2af0976caba7474