Allow destroying display content on removal

This CL sets the behavior for displays when they are removed.
For public displays by default all content will be moved to the
primary display and become focused. For private displays default
behavior is to destroy all content - first it moves stacks from
the secondary display to the primary display to the bottom, then
it destroys all activities in those stacks.

This CL adds two specified behaviors as modes, so in future these
rules might be altered if needed.

Bug: 34263289
Test: android.server.cts.ActivityManagerDisplayTests
Test: #testContentDestroyOnDisplayRemoved
Change-Id: I3f89f06ff82cb4b487df58a86ba3b146a32cbd00