android: add IceServer.urls field

This makes api more consistent with ios and native library

BUG=None

Review-Url: https://codereview.webrtc.org/3012843002
Cr-Original-Commit-Position: refs/heads/master@{#19770}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0ea0310b89a35e20072b3cad6712c46eba869228