Add a callback to TaskStackChangeListener which gets fired when the system
might need to inform the user that a specific app might not work in
multi-window.
Use that callback in SysUI to show a translucent activity which scrims the
activity behind to inform that it might not be resizable.
Debounce the information to once per multi-window session, to not make it
annoying.
Introduce launchTaskId to start an activity in an existing task, and protect
that with START_TASKS_FROM_RECENTS permission.
Bug: 27327287
Bug: 27431869
Change-Id: I89e8d653872ab01ba3c1e252b426e5481da0e6ca
2adba07d75