Set up demo mode repository for mobile icons

This CL is the sketch of how we can achieve demo mode using the modern
architecture that makes up the new data pipelines. The basic idea is to
put a `RepositorySwitcher` in between the real repository impl and the
demo mode repository.

RealRepository
              │
              ├──►RepositorySwitcher──►RealInteractor──►RealViewModel
              │
DemoRepository

In this case, we have a `IRepository` interface which both the demo and
fake repositories implement, and the switcher decides which
implementation to connect to the (real) downstream classes based on
whether or not demo mode is active.

Note that this CL does not hook up the demo mode repository properly,
since the old data pipeline uses a demo icon group layered on top of the
real one to achieve fake icons. That makes things kind of a mess and so
will be implemented in its own CL

Test: DemoMobileConnectionParameterizedTest
Test: DemoMobileConnectionsRepositoryTest
Test: MobileRepositorySwitcherTest
Bug: 249790009
Change-Id: I9ad6bab6dd0ab6eadf43232e3bea0f4fa3116a17