Migrate to DashboardFragment in ToggleFeaturePreferenceFragment

Root Cause: ToggleFeaturePrefreferenceFragment becomes heavy when more and more features come in.

Solution: Mirgrate to DashboardFragment, a plugin-style preference controllers, can help us separate the preference and its logic into its own controller.

* This is the first step for the whole migration. Change to extend DashboardFragment and fill up the missing override functions.
* Reorder fields and functions to the recommended general approach.

Bug: 171272809
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=com.android.settings.accessibility

Change-Id: I852091700a4b3b3c7bbdbc82f0b5dc47c087d61c