Introduce utility method in CarrierConfigManager to get config subset

This CL introduces a hidden utility method for system components to
get the subset of carrier config.

Many system components has been update to use the new
CarrierConfigManager API to get subset of config. But the callers
side have repetitive private methods to do the same thing. The
utility method in CarrierConfigManager will remove the duplication.

Bug: b/263267340
Test: FrameworksTelephonyTests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:96727486197e8c0602ffb8f27787aae1b256fa44)
Merged-In: Ifbd84c67525fd9ea2925e5b735788e6c3c22dd34

Change-Id: Iac88c03f2c9b4ac80dd2ed0b72a200cfc1631d21