Set oom adj for processes displaying app-overlays to PERCEPTIBLE_APP_ADJ

For processes with a window of type TYPE_APPLICATION_OVERLAY adjust their
oom importance to PERCEPTIBLE_APP_ADJ to reduce the chance of them getting
killed by the low-memory-killer since they are displaying something that
is perceptible to the user.

Also z-order TYPE_DREAM windows above alerts windows.

Bug: 33256752
Test: cts/.../run-test CtsAppTestCases android.app.cts.AlertWindowsTests
Change-Id: I4c05a9fee6fad61399bf4d10c8647467cc596ca6