Ensure default handler always is set if no other error handler registered

To avoid memory leaks we set the error handler to null in activity
onDestroy. But, the agent may receive callbacks after this, so
ensure at minimum that we set the default logging handler to default
handle errors post activity destroy.

Bug: 18200874
Bug: 18202261
Change-Id: Ie7251a149ab02c474d8affd15af6b9ea48c2cdc0