Move IPC calls to background thread in CarrierTextController

In CarrierTextController, move call to ConnectivityManager to the
background thread and cache its value. Move handleSetListening
completely to background thread (make sure that callback call is posted
in main thread).

Also, get KeyguardUpdateMonitor only once, not necessary to keep track
of whether we can listen to calls (as we have a cached value now).

Test: atest CarrierTextControllerTest
Test: manual (start listening), does not trigger StrictMode
Fixes: 140034799
Change-Id: Iaf0771e8d71e07212998f44d73465b1a81b2e268