Support accessibility shortcut secondary action (7/n)

Implements sharedPreference to keep the user preferred shortcut for each
service.

Bug: 142530063
Bug: 142531156
Test: make RunSettingsRoboTests ROBOTEST_FILTER=ToggleFeaturePreferenceFragmentTest
Test: make RunSettingsRoboTests2
Change-Id: I2a7c54ddc32764f6d88f2efed87982d469acc183