Set importance for processes displaying app-overlays based on visibility

We were previously setting this based on when the window was added and
removed. This can lead to issues where an app isn't allowed to show
alert windows so window manager hides the window after the permission is
revoked, but the window still exist in the system so the process
importance will still be high. We now set the process importance based
on if it has visible window surfaces or not.

Bug: 33256752
Test: cts/.../run-test CtsAppTestCases android.app.cts.AlertWindowsTests
Change-Id: I4e406a94683ec0eecc0911d0195e641c693e1c4a