Redirect Value Source of SignalStrength Constants

SignalStrength contains a few hidden constants that
are identical to publicly exposed constants in
CellSignalStrength. Since the CellSignalStrength
values are public, redefine SignalStrength to point
to the public values and limit access to them
(since there is already a public alternative).

Bug: 30188945
Test: compilation
Change-Id: Ie7f295ef4d7849f7ce3a631cbdffe3d6670da5bd