Rename getApnTypeString

Rename to getApnTypeStringInternal for compatibility since we don't want
the exposed API returning a string like "Unknown".

Also add a unit test for the method converting between APN bitmasks and
strings in TelephonyRegistry.

Bug: 173133122
Test: atest TelephonyRegistryTest
Change-Id: Id5a0aa8bc67b65c87e4e37ee8b070c9664f9e6d0