Cache OverContext to prevent deadlock

There may be a deadlock if WMS is obtaining DisplayInfo during
creating SystemUiContext on seconday display
while PMS is querying current power state from DMS.
This CL caches the OverlayContext used in LocalDisplayAdapter
to prevent DMS from holding ActivityThread lock.

fixes: 227308239
Test: atest LocalDisplayAdapterTest
Change-Id: I41c44ea070673f048b49d108e029ae90d96645ad