Update the split screen APIs used by the launcher

Makes the following changes:
1. Adds a boolean visible parameter to the onTaskChanged method of
SplitScreenListener to allow the launcher to determine which task is
on top of a stage
2. Allows the launcher to specify a fill-in intent when asking to open
a PendingIntent on the main or side stages.
3. Allows the launcher to ask to remove a task from the side stage.

Bug: 179176511
Test: manually tested against the foldable taskbar launcher.
Change-Id: I9fa530f546af58779ccf71d0753d9b6d3479fd0b