Add feature flag to define system API for IMS service.

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

Bug: 365488868
Test: build pass
Flag: com.android.internal.telephony.flags.support_ims_mmtel_interface
Flag: com.android.internal.telephony.flags.support_isim_record
Change-Id: Idf3d847fcdf2348a09befa9e30fd4cbffad6e282
2 files changed