Allow input to fall through custom caption region

Make the caption window in Desktop Mode a spy window when the
transparent caption flag is requested. Then, when MotionEvents on the
caption bar, issue cancellations if the gesture did not start within the
bounds of the customizable space and the gesture exclusion bounds
reported by the window.
This allows input to fall through uncancelled to customizable app
content as long as they've placed it in non-occluding caption regions
and they requested to take over input in that region. Otherwise, the
system will handle the events as a caption gesture (drag, double-tap,
etc).

Other window decoration types (app handle, non-desktop freeform caption)
do not support this feature, so their caption window is not spy.

Bug: 316387515
Test: atest WMShellUnitTests
Change-Id: Ibb966a15fa3befe1f1dec802e448a973671ac645