Don't allow sandbox uids to call registerReceiver

Adds a check to ensure that the uid that calls registerReceiver
is not an SDK sandbox uid. This will disallow the SDK sandbox from
dynamically registering to receive broadcasts.

Test: Manual. Set up sample SDK provider to receive TIME_TICK
      broadcasts. Note that broadcasts are received every minute
      without this change, and are not received with it.
Bug: 209770512
Change-Id: I2edec36d9579287e8458a3f13228b1bd1298cf41