Empty DC's apnList when the DC is free.

Not doing this causes DC.apnList's to have multiple entries in the
apnlist which causes DC's to look like its connected to both IMS and
INTERNET. And when selecting CDMA only mode on a Xoom we will not bring
up the connection thus no data connectivity.

A "cherry-pick" from frameworks/base/ b44b13a3f5967a7ce80f7124dc7b9c486f49c275

Bug: 6939338
Change-Id: I49881adb2710d7fe5d88c3aef748618bda7d65f1