Create Central Version of dataStateToString()

There are currently two slightly-different copies of
dataStateToString().
-TelephonyManager#dataStateToString()
-TelephonyRegistry#dataStateToString()

The TelephonyRegistry version doesn't have a string for the
DISCONNECTING state. In order to have a single copy that can
be accessed from TelephonyRegistry (not mainline) and TelephonyManager
(mainline), move both copies to TelephonyUtils.

Bug: 149717408
Test: make; atest TelephonyRegistryTest
Change-Id: I36945a8a432f7b4f0c449f7414996372e852b5b8