Consolidated all data allow/disallow logic into one method

Consolidated all data allow/disallow logic into one method
and got rid of the obsolete isDataPossible. This is the
preliminary work for checking if data is allowed just right
before data connection entering connected state.

Test: Telephony sanity tests and unit tests
bug: 33847610
Change-Id: Ida89d454d984f7f0e4af8b20739dc68c0512e63c