Factor out telephony permission checks into a helper class.

The aim is to cut down on duplicate code (for a particularly high-risk
area) and make it easier to expand READ_PHONE_STATE access to
carrier-privileged apps in a follow-up CL.

No major functional changes are intended, although some logging will
change slightly (different tag / less verbose carrier privilege
logging without DBG flag).

Bug: 70041899
Test: bit FrameworksTelephonyTests:*, nothing visibly broken on boot
Change-Id: I3b2aa9b8d6177a25d532060009508ef31baea69c