Allow data auto attach during voice call.

During voice call, if voice rat is 2G / 3G, we would allow data auto
attach to make sure we try best effort to switch data to it, if needed.

Bug: 134981077
Test: manual
Change-Id: Ie5a810f013cfb75ed1ecc9edce2794abd73f1540
Merged-In: Ie5a810f013cfb75ed1ecc9edce2794abd73f1540