Fix threading issue for advertising.

Post callback to main thread to execute. In general we should avoid
running app's callback method from binder thread as the callback method
may block.
Also removed callback from advertisers on stop advertising callback.
This fixes the issue of not being able to restart adv for limited
advertising.

Bug: 17045567

Change-Id: I56cd2bdf4b1ad120a0358fa4065ad77be4bff144