Add Nullability Annotations and verification to CellIdentity

Bug: 148177166
Test: atest FrameworksTelephonyTests
Change-Id: Ie98db2ce393a76644a4773045eafc28ce6f4a4cb
Merged-In: Ie98db2ce393a76644a4773045eafc28ce6f4a4cb
(cherry picked from commit 33f62a071c2e128379ce2b2c41bdadfeca1740ed)
7 files changed