Added key Magnification capability

To support window magnification, we use this key to store magnification
capability. If the device is capable of only one magnification mode,
it can not switch the magnification mode between full screen mode and
window mode.

Bug: 144327603
Test: atest SettingsProviderTest
Change-Id: If5aaa1ccc851f675afed204a367f08224e04df9b