Add API to Retrieve Detailed SignalStrength Info

This commit adds an API to retrieve detailed
CellSignalStrength from a SignalStrength instance,
which permits granular attribution of signal strength
details and avoids proliferating duplicate code up to
the SignalStrength class (which ideally never needs to
changed again except when adding a new technology or
if we add CarrierAggregation support).

Bug: 118166143
Test: wip
Change-Id: I6bbd86ec460c3cff84f1a22348381b15c66c1065