Refactor SettingsContentObserver and add it to follow typing feature.

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