Add new APIs to support RegionInScreen in A11yWindowInfo

To support non-rectangular visible area for windows, add new APIs
about RegionInScreen in A11yWindowInfo to represent the actual
interact-able area of a window.

Bug: 132146558
Test: a11y CTS & unit tests
Change-Id: I86bb6bc8c567e09f01a3f853a3cffd896ce934c8