Set subscription_mode in the database to a default value

If ro.telephony.default_cdma_sub is set, use that to set the
default CDMA subscription source in the database. If that is not set
use the default preferred value defined in the source code.

Change-Id: I11fff596a5fe721c64f192c889672326517dc43d
Merge branch 'cm-10.2' into martincz