Remove hotspot receiver from manifest.

The hotspot receiver if used to cancel any tethering alarm set when wifi
ap state changes. Removing it from the manifest, and only register the
receiver when we schedule the tethering alarm.

Change-Id: I97c3be6e2a374949208bbdd4ac5ccc2fabf0291a
Fix: 35968322
Test: make RunSettingsRoboTests