Adds power repository and interactor.

UserSwitcherActivity refactor: CL 3/7

Adds a repository and an interactor to expose screen on/off state to the
rest of the app. This is needed by the user switcher as it finishes the
activity if the screen turns off.

Bug: 243844359
Test: Included unit tests.
Change-Id: If428cae7be2c25639d43a56717b7e6e9f0b86bc5
7 files changed