Add callback hack to find out when to reload system properties.

Every IBinder object can accept a new transaction to tell it that
it might want to reload system properties, and in the process
anyone can register a callback to be executed when this happens.

Use this to reload the trace property.

This is very much ONLY for debugging.

Change-Id: I55c67c46f8f3fa9073bef0dfaab4577ed1d47eb4