Add getCurrentImeTouchRegion to the WindowManager API

This is a followup to ag/2370980/ to expose the API more
externally as a WindowManager API. This allows application
access without needing to reflect on IWindowManager.Stub.

We require this to get the current IME touch region to
display it properly in VR contexts.

Test: Manual
Bug: 62194867

Change-Id: Ibbf336e1e473b65d4c76a1492da187d50cfda424