Expose NRI#rejectCause as Public

Expose the existing rejectCause() API as public, which
is useful in situations where a caller was unable to
pre-register for TelephonyCallback#onRegistrationFailed().

The reject cause also indicates a terminal failure, which is
easy to distinguish if a caller is not interested in
intermediate results during a network selection procedure.

Bug: 239730435
Test: atest NetworkRegistrationInfoTest#testGetRejectCause
Change-Id: I177f940d7405636a326dda9a135f6427755bff46