Extract the logic of MagnificationMode to the controller

To move the preference to the upper layer, we extract all logics
to its controller.

We move all tests to the test files and add server tests to
verify the behaviour of edit shortcut dialog.

Bug: 182992338
Test: atest  MagnificationModePreferenceControllerTest

Change-Id: I34c4361e2e116a22c3e34bd35c8ac8cac752ab96