Add API to handle user's subscription selection changes.

When user selects a eSIM subscription, Settings app informs Telephony.
Telephony needs to take actions such as writting it into global
settings, switch profiles and notify registrants, for example
AlternativeNetworkAccessService.

Bug: 120945564
Test: unittest
Change-Id: I846d9444aac368d183e06744c9eb8aa0c08dfe6a
Merged-In: I846d9444aac368d183e06744c9eb8aa0c08dfe6a