Allowing enabling new data by device config

cl/421423121 successfully pushed the default
flag to all T devices. This CL allows ramping
up new data stack from server side.

Note this is just the temp code. After 100%
penatration rate, this flag along with all
legacy data stack modules (e.g. DcTracker) will
be completely removed.

Bug: 204124694
Test: adb shell device_config get telephony enable_new_data_stack
Change-Id: I70acc0df688ba6b8a87235e81089792719476278
Merged-In: I70acc0df688ba6b8a87235e81089792719476278