Grant SYSTEM_ALERT_WINDOW during screen capture.

While an app has an active screen capture MediaProjection, enable the
SYSTEM_ALERT_WINDOW app op for it if it requests the permission and
the user hasn't explicitly allowed or denied it. This enables the app
to use TYPE_APPLICATION_OVERLAY windows to show custom UI on top of
what is being captured. Needed for live-streaming apps.

Test: Manual, using a hand-written fake recording app and a real one
Test: atest MediaProjectionTest
Bug: 148002116
Change-Id: I503f3223608627c80520e98be840f793efd53d2a