Multi-shade foundation - data layer (1/5).

Early foundation for the multi-shade framework.

Data layer for the multi-shade foundation. Includes cross-layer shared
models, a proxy for sending touch events into the framework, and a
repository to store and retrieve application state.

Bug: 272130181
Test: includes unit tests for code in this layer. Tested end-to-end in
the last CL in this chain.

Change-Id: Ic1dfd8deb0bdde95288d824f91986e4749810ca2