SF: Replace the active display terminology with front internal display

Active display is ultimately in service of foldable displays. As we
move to remove and replace the active display concept, we should make
the relationship clear.

Rename the following members of SurfaceFlinger:
* onActiveDisplayChangedLocked()
* getActiveDisplayDevice*()
* sActiveDisplayRotationFlags
* mActiveDisplayTransformHint

Flag: EXEMPT renaming
Bug: 255635821
Test: presubmit
Change-Id: Ifada4a46974b0e24be6b9e8cf6ae675ceb48dbc5