Add feature flag enable_rtt_hold_carrier_config.

Design Doc: go/rtt-hold-unhold-config-in-android-connectivity-design-doc

Bug: 413765645
Test: build pass
Flag: com.android.internal.telephony.flags.enable_rtt_hold_carrier_config
Change-Id: Iaeffabc672b78fd7c79b621c21596e2f8daaf607