Bug: 21589105 Rescoping SYSTEM_ALERT_WINDOW...

AppOpsManager:
Changed the default mode for SYSTEM_ALERT_WINDOW to MODE_DEFAULT instead of
MODE_ALLOWED. Otherwise, an app that did not declare for this permission will
actually be allowed to perform OP_SYSTEM_ALERT_WINDOW, which is undesirable.
This change also allows callers to make their own decision based on the
current policy (M vs pre-M apps).

policy/PhoneWindowManager:
Added additional checks that will handle MODE_DEFAULT - this happens when an app
is newly installed but not yet configured.

wm/WindowManagerService:
Enriched some checks to include the treatment of MODE_DEFAULT - this will allow
pre-M apps uninterupted capability to draw on top of other apps.

Change-Id: I8de77730e158c97587427820cfba721bd5607bea