launcher3: Improve SimpleBroadcastReceiver.

  Unless explicitly annotated, parameters in java are by
  default nullable. There are a few cases where a null context
  may be passed to the unregisterReceiverSafely function
  of SimpleBroadcastReceiver.

  To mitigate misuses or incorrect contexts being passed for
  register vs unregister, keep the context as a strong reference
  in the constructor.

  Also added NonNull annotations for any public callsites to
  enforce behavior.

Bug: 395019017, 395966395
Flag: NONE - bug fixed
Test: manual - presubmit
Change-Id: Ie371fa45cadceaf51cf184b446df9123ef27c337