Add carrier config for allowing merge of RTT calls

Add a carrier config key that specifies whether merging an RTT call is
allowed.

Bug: 136035168
Test: manual

Change-Id: I8cfdf68c69b50abed88d7c835db0480a53755f5a
Merged-In: I8cfdf68c69b50abed88d7c835db0480a53755f5a
1 file changed