Add feature flag to public API for IMS service.

This commit adds a feature flag to define public API for IMS service.
These flags allow use of specific APIs as public API.

Bug: 359721349
Test: build pass
Flag: com.android.internal.telephony.flags.support_sms_over_ims_apis
Change-Id: Id5c72a0550859a3609dd47efade3958f1f21f193