Use carrier key from carrier config

When no key is available, check if a public carrier key is stored in
carrier config as a backup for the carrier.

Bug: 175801497
Test: manual and atest TelephonyManagerTest
Change-Id: I8e855cca847d65276bfb34a797f5a2fd076a0763
Merged-In: I8e855cca847d65276bfb34a797f5a2fd076a0763