fix coordinate calculation of TouchExplorerTest

Currently, getRightSideOfScreenRegion is using display size to calculate
coordinate of right half of the target Activity.
This makes this test fail when the application is not full screen.
Fix it to use window coordinate.

Bug: 163435521
Test: TouchExplorerTest.testGestureDetectionPassthrough_initiatesTouchExploratio
Change-Id: I971eb3fc82c3dba63d62bbf21c04f5ba70fc4f39