Refactor SubscriptionManager caching code

Create a genericized class to use for SubscriptionManager caching calls
in order to avoid duplicating logic that fetches values from ISub.

Bug: 151953109
Test: atest android.telephony.cts.SubscriptionManagerTest
Change-Id: I6682ded8aec8cb3e50521584c177df6d5dae8c49
3 files changed