Fix lifespan with "Don't keep activities" on

- This option simulates low memory states where non-visible activities are destroyed rather than backgrounded. Well-behaved apps should pass this test without issues.
- Required changing fragment creation, and properly handling onDestroy to
  avoid tons of warning messages in the logs
- *May* be a serious issue on old devices, which actually do destroy activities more aggressively.
- Previously, because we didn't have proper lifecycle management, the main activity was set in the manifest to not re-create on orientation changed. Removed this requirement.