Cache nrBand during idle

Without the change, the nrBand change during idle is ignored, hence even after timer ended, the device still shows the earlier cached nr band.

Fix: 395503205
Test: verified by reporter
Test: basic voice call + data browsing
Flag: EXEMPT bug fix
Change-Id: I84a5a06b1969f4704887b85c7d631d67b9d9adef