Fix thread leak from RttTextHandler

The reader thread for reading from in-call wasn't shutting down properly
before. This led to interleaving of messages if RTT went on, then off
again, then on. Add an interrupt call upon teardown so that the thread
can properly stop.

Change-Id: I1f0b4afe14ad218ca58e2feb3d8861f5d76ebe8e
Fixes: 110570772
Test: manual (follow repro steps in bug, but type really fast),treehugger