Migrate EmptyShadeView to recommended architecture

This is a step towards making the empty shade text react to changes in
Modes instead of just mentioning DND.

Bug: 366003631
Bug: 308591475
Flag: android.app.modes_ui_empty_shade
Test: EmptyShadeViewModelTest & tested manually with flag on and off

Change-Id: I45ab6ad5d6c85f13891a9fdcdfdff8e69068d783