Guard absent SplitScreen in PipTaskOrganizer

Current code assumes the presence of SplitScreen, so it crashes if
SplitScreen doesn't exist.

Bug: 207495281
Test: Transition in and out of PIP still works on phones.
Test: Transition from PIP to split screen on phones.
Change-Id: I6f2f05a1237c9e64f1df95b05347a1affa9b5a27
1 file changed