Baseline Testables updates for Ravenwood.

The `testables` library offers utilities like `TestableContext` and
`TestableLooper` that are heavily used by SystemUI tests.

As part of onboarding SystemUI tests in Ravenwood, we need these
testables to have baseline functionality, even though Ravenwood
doesn't have full `Context` support yet, which we do by quietly
deferring features (like SettingsProvider interactions) when
running under Ravenwood, along with TODOs to circle back as we add
more support under Ravenwood.

For `TestableLooper` we need to defer any `Looper.getMainLooper()`
interactions until the actual test attempts to use the Looper, since
the RavenwoodRule won't have created a main looper before a test
is running.

Bug: 319647875
Test: atest SystemUiRavenTests
Change-Id: I0e38acef814bf9baa0fe61c5478f77a555f312b6