WhisperPush: Get a new registration id on each request.

    - Users who restore WhisperPush data across devices can
      end up in a bad state where their new device will not
      receive messages while the old device continues to.

    - Instead of fetching from shared_prefs, get a new
      gcm id on each request. Ideally this would be done
      weekly/more frequently by the application.

Change-Id: Id4baf5618579d0bb1b0e656d2bcb7235df5e41e6
(cherry picked from commit 735476cfc85b23551e58011eee9724c3cbcb45fd)