Create a method to allow child classes to define it's main switch toggle's pref key

We'd like to make the main switch toggle's preference searchable. Since we construct the preference programmatically, this would allow the preference to be automatically scrolled to when the user is brought to this page from a search result.

Bug: 372171003
Flag: EXEMPT refactor
Test: manual
Test: atest ToggleColorInversionPreferenceFragmentTest
      atest ToggleDaltonizerPreferenceFragmentTest
      atest ToggleFeaturePreferenceFragmentTest
Change-Id: I6eed7598ad296a758573318cf4bde5de30c7005c