stop polling service state when shutting down

if pollServiceState is modem triggered,  we will keep polling state even
radio is off. This will cause "No service" and "No SIM card" hangs here
for a while during the power off sequence.

Bug: 34359617
Test: Manual
Change-Id: I5b9369601ca11b03223e22dabd60672130bea26c
Merged-in: I9a6f0412bdf7dd67316ce982834a7ad43054f7f4