Offsets an additional distance for requestRectangleOnScreen.

In previous, requestRectangleOnScreen would only apply the shortest
movement on the magnifier to the rectangle. However, it will make typing
words out of the screen. Therefore, we add an additional distance for
moving the magnifier to help a user know the surrounding context.

Bug: 148882866
Test: atest FullScreenMagnificationControllerTest
Change-Id: Iee7a4f13395ed8acf3ffa7df759542fcb08190f8