Implement magnification switch button (2/n)

This change implements showMagnificationButton and
removeMagnificationButton methods. In the later changes,
these methods will be called when the magnification capability is
ACCESSIBILITY_MAGNIFICATION_MODE_ALL and the magnification scale is
changed.

Bug: 145780606
Test: atest ModeSwitchesControllerTest IWindowMagnificationConnectionTest
Change-Id: I4f6862881ff8a5ab4bbf8aa1346a72364db57321