Makes the movable region of magnification button sticking to the edges

When dragging magnification button, the button should stick to the
closer edge of the screen edges automatically after the dragging
gesture is finished.
And not to reset the position after the button is removed. So the but tton
position can be recovered after calling showButton() again.
Demo video: b/162706034#comment3

Bug: 162706034
Test: atest MagnificationModeSwitchTest
Change-Id: I78127f3e3f479e797426e0f451424f6ed090cd78