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: I431deca8cb72e34c105af7b65f97c8c785cb5a1a