Stop background Thread in Phone app

There was a bg Thread running forever after opening the Phone application.
onPreDraw() gets called after the thread has been killed and after the
app has exited, so it creates a new thread.

Store separately whether we still have to process requests or not.

Change-Id: I15a08bc8c0c2eb982f580dbc41b1cfa2c0121f7b
Signed-off-by: Gergely Szell <sethyx@gmail.com>