make transform hint multi-display aware

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