Avoid NDC deadlock when re-arming network alert.

When NDC delivers limitReached() event, NMS re-arms the alert, which
makes another call back to NDC.  If NDC already started processing
another call, this causes deadlock.

Fix this by re-arming on a handler thread.

Bug: 5879848
Change-Id: I46fa78fe1f1122c5f31dd6020483e5ae89a9f39d