Remove special case handling for letterboxBounds.

InputWindowHandles for the letterboxes are already provided by the
WindowInfosListener callback, so we can directly use those in the various
calculations in these files. However, those handles have no IWindow
token so we instead look for non-empty frame & non-empty touch region
to decide which windows to include in calculations.

Windows without an IWindow token are still excluded from being passed
back to accessibility services.

Fix: 199358388
Test: open app in letterbox mode, use logging to check that
      AccessibilityController stops iterating once the letterbox
      windows fill up the remaining screen bounds
Test: atest CtsAccessibilityServiceTestCases
Change-Id: I625afa17711fcd52bbb7fe9a9ba51500eb1bef5c