SystemUI proxy updates for managing split screen

Allow the launcher to provide a fill-in intent when starting a
PendingIntent splitscreen, and add a new method to allow the launcher
to request removing a task from the side stage.

Bug: 179176511
Test: Manually tested with the foldable launcher prototype
Change-Id: I23173b432ffe9c988744641170412aa9111cf55d
1 file changed