telephony: fix mobile data disconnect during voice call

refer: http://review.cyanogenmod.org/#/c/50800/

Voice technology is not properly supported in older RILs. So
use data tech in isConcurrentVoiceAndDataAllowed().

When using RIL blobs, the value would be set to UNKNOWN,
resulting in data being disconnecting during call despite
being on 3G.

When using source-built RIL, the value would be set to
UMTS regardless of 2G/3G status. (At least from reading
the source.)

Change-Id: Ic82d5e362ba403a432c2760bbb931d72eae7b776