Update ice server provider response format in iOS AppRTCMobile

BUG=webrtc:5963

Review-Url: https://codereview.webrtc.org/2627523004
Cr-Original-Commit-Position: refs/heads/master@{#16050}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: cc882afdf79f5a8235887f59fcb842998a62131f
7 files changed