Refactor SettingsContentObserver and rename it to
AccessibilitySettingsContentObserver. Besides, we register it to observe
follow typing feature preference value.
The reasons behind refactor:
1. We change callback signature due to the consistency for register it
by preference key, not by Uri.
2. We refactor the default preference key to a seperate method. Since
the default value is related to accessibility, we rename it with
accessibility prefix.
3. We can register different callback for difference collections of
preference keys.
Default preference keys: They existed in the previous constructor.
1. Settings.Secure.ACCESSIBILITY_ENABLED
2. Settings.Secure.ENABLED_ACCESSIBILITY_SERVICES
Bug: 194668976
Test: make RunSettingsRoboTests ROBOTEST_FILTER=
AccessibilitySettingsTest
AccessibilitySettingsContentObserverTest
AccessibilityShortcutPreferenceFragmentTest
MagnificationFollowTypingPreferenceControllerTest
ToggleFeaturePreferenceFragmentTest
ToggleScreenMagnificationPreferenceFragmentTest
Change-Id: Iafd27e044ebe2536ae7ae55c1c80af54f7f0f822
