Move screenshot receivers and add tests

Moves the various broadcast receivers out of GlobalScreenshot and
adds unit tests for them, along with some other changes to make
them more easily testable:
    - ScreenshotSmartActions is now a stateless injectable class,
      instead of a collection of static methods
    - DeleteImageInBackgroundTask removed, in favor of just calling
      a background executor directly
    - remove the TargetChosenReceiver (used to remove notifications
      after a share target is chosen) since we're not using
      notifications anymore

Bug: 160325487
Test: atest SystemUITests, plus manually checked that screenshots
continue to function as expected

Change-Id: I1c054dddd76404f385e59f7ab7317beaafde1106