Moving some of the common bits of Sidecar and Extensions
into helper classes and adding both to the reference repo.
Bug: 154172225
Test: m androidx.window.extensions
Test: Manual, by including extensions in the emulator image
Test: gradlew window:window:cAT
Change-Id: I44ff06b06b2202a5d687216e017528098dc14d1d
336f9fdf86