Multi-shade foundation - UI layer viewmodel (3/5).

Early foundation for the multi-shade framework.

View models in the UI layer for the multi-shade foundation. Includes two view-models:
1. A top level MultiShadeViewModel to coordinate between the individual
   shades and the scrim
2. A ShadeViewModel to describe UI state for a single shade

Bug: 272130181
Test: includes integration tests for code in this layer together with the domain and data layers. Tested end-to-end in
the last CL in this chain.

Change-Id: I85ff1c24a780ffae66a7e352cf75acdbdbc749ba