Create settings for window magnification (3/n)

This change is for internal test.
Add a switch preference for magnification enable mode.

Bug: 146019459
Bug: 146473544
Test: make RunSettingsRoboTests ROBOTEST_FILTER=MagnificationEnablePreferenceControllerTest -j64
Change-Id: I85fdbb5b0efc106d5301d95802b8d6b7dbe8145d
7 files changed