Fix inconsistentcy API in CellInfoNr

Add the annotation CovariantReturnType to
CellInroNr#getCellSignalStrength() and CellInfo#getCellIdentity() to
modify the return class to a NR specific class.

bug: 136514894
Test: atest CellInfoTest
Change-Id: If87b6bf13a84c60632a526ad9c49ca0ca77b5f8f