if a layer is not mirrored, we now use its display
as the source for the transfrom hint calculation
instead of always using the default (main) display.
this change does two thing:
1) we make updateTransformHint take a DisplayDevice
as a parameter instead of hard-coding the
main display.
2) each time we do a transaction that could change
the hint, we go through all layers and
figure out which display should be used for their
transform hint.
Bug: 7599344
Change-Id: I9b04a95e6c372dd770bacf81d8ef6f8e31b87b83
8430095879