Make BackEvent#getTouchX and BackEvent#getTouchY floats.

Bug: 228533817
Test: Try swipe back to home on Flag Flipper (with Predictive Back
Animation toggle enabled in Flipper) and make sure animation looks good.
Try in both landscape and portrait mode.

Change-Id: If2165827a0f66f8e0bd8f78483577c3ac4c9f213