Add flag for displaying non-user's Windows to user.

Created a new flag that indicates that a window should be shown
to all users. For the flag to be valid the owner of the window
must have system permissions.

Also separated system window types into those that show to all
users (e.g. StatusBar, Keyguard, ....) and those that appear only
to the owning users (e.g. Drag, ANR, TOAST, ...). Those that appear
only to their owner can override their default behavior using
the new flag (e.g. LowBattery).

Fixes bug 7211965.

Change-Id: I1fdca25d57b7b523f0c7f8bceb819af656c388d4