chore(#AlwaysOnMagnifier): Replace string constant with Settings Pref Key

In AccessibilityManagerService, previously we use string constant as the
name in Settings getter method. After the Settings key is defined, we replace
the string constants with the defined key.

Bug: 146504200
Test: atest AccessibilityManagerServiceTest
Change-Id: Ia31a068f9e882b2f5e5a1a3112b682006ee54fe1