Updates magnification button after magnification mode transition is done

Magnification button may be incorrect after magnification mode
transition is done. So just update magnification button with the target
mode after magnification mode transition is done.
The new UI behavior would keep magnification button showing when
the magnification mode transition is happened.

Bug: 181000039
Test: atest MagnificationControllerTest
Change-Id: Icbe34aead2e63c8a65c3238bdc62ebb30974ac6a