Expose DataFailCause as public API

API Council requests to add document for public API getLastCauseCode()
which return DataFailCause constants. DataFailCause constants are widely
used in system to report data connection failure code. But currently it
is system and hide API.

By opening DataFailCause, we can have better javadoc for both system and
public APIs. This also benefits applications that need to refer to the
consistent data connection failure code.

Bug: 147804257
Test: atest com.android.internal.telephony.dataconnection.DataFailCauseTest
Change-Id: I27bc1d88ba0e7b25fe8f032df1f4d14b805a68bf