Camera: Don't switch image callbacks during extension sessions

- Relying on camera capture callbacks to switch appropriate
  camera output surface image callbacks can result in timing
  issues. For example a capture started callback from a new
  capture request can execute before the sequence complete
  callback gets scheduled for the old request. This can result
  in images from the new request getting redirected to the
  wrong/old callback.
  Avoid this by using a single image callback instance per
  camera output for the entire duration of the capture session
  and allow individual images to be redirected to appropriate
  handlers on a precise timestamp basis.
- Additionally refactor an odd looking if-else case within
  'ImageWriter'.

Bug: 180451857
Bug: 180451435
Test: Camera CTS

Change-Id: Iae497ff2eefacc00ea45ff5e598581156aa64c0d