Update hashCode in CellSignalStrength classes

The CellSignalStrength hashCode function was using a
fairly ineffective method of hashing. An External
reporter requested that we fix it. This CL moves to
using the Objects.hash() implementation.

Bug: 22479413
Test: compilation
Change-Id: Ic017ba54ef757fd3ec3e5000ac61108dd836bd8a
4 files changed