WM: Refactor DisplayContent.getOrientation in preparation for DisplayArea (2/n)

DisplayContent.getOrientation:
 - When the display is frozen, we no longer need to keep track of the last
   non-app-window requested orientation, if any, because the case of the
   keyguard becoming intermittently unoccluded is now handled by the condition
   just below.
 - We no longer ignore the above app window containers while the display is frozen.
   This was originally introduced so we don't pick up the keyguard's orientation
   during occlusion, but this was no longer necessary once we started tracking the
   occlusion state explicitly.
 - The logic for forcing the keyguard orientation whenever it is showing or we're
   unoccluding is now moved to the NonAppWindowContainer, in preparation for
   DisplayArea.

NonAppWindowContainer.getOrientation:
 - Cleaned up looking for the orienting view when Keyguard is going away

Bug: 147406652
Test: atest WmTests
Change-Id: Icb0b6f3e80d26a5ce060f2379ca0ca26f6146bbf