Set theme for Screensaver

- Set Theme.DeskClock for Screensaver so that app-level theme attributes
  such as ?colorAccent can be resolved succcessfully.
- Also did some minor cleanup of Screensaver and ScreensaverActivity.

Bug: 35330855
Test: manually verified that crash does not occur when using Screensaver
or ScreensaverActivity on API 19 and 25 devices.

Change-Id: I107fdd51af5a97f41464942dc3e2f7730f11b5c2