Fix empty signal strength bar shows after turning radio on

Either APM off or modem SSR happened, the signal strength
indication might come earlier than RAT changed and radio state
changed to on, which is result in reseting the current valid
signal strength while starting to poll network state under radio
power off, then leading to show signal level 0.

To fix it, query the signal strength mainly if deteted to APM off.

Test: manual
Bug: 173993624

Change-Id: I21e0f7fd1552493b53b03350065459f81aa3ffb8