ZenModesListAdd extend BasePreferenceContoller

Makes ZenModesListAddModePreferenceController extend
BasePreferenceController. This should fix warning statements, and has
the effect of adding "Create your own mode" to the Settings Search menu.

Test: build+flash
Bug: 361144180
Flag: android.app.modes_ui
Change-Id: Ic8675420b831818f658506f36938f38e803cc9aa