Fix lifespan with "Don't keep activities" on
- This option simulates low memory states where non-visible activities
  are destroyed rather than backgrounded.
- Required changing fragment creation, and properly hanling onDestroy to
  avoid tons of warning messages in the logs
'Keep screen on' pref requires recreating the main activity
Added back the singleTask launch mode
Changed the keepscreenon pref toggle to not require restarting the
activity
Merge pull request #964 from garvankeeley/dont-keep-activities

Fix lifespan with "Don't keep activities" on