Organize multi-window enabling

Use AMS#mSupportsMultiWindow to indicate if some form of multi-window is
supported.
- If no multi-window implementation is enabled, this will be always set
to false.
- If a corresponding config is set to false, it will disable all forms
of multi-window.

Bug: 38173505
Test: android.server.cts
Change-Id: I7efe07e2fe073891dd55844979156831097c28b7