Add Class-Filtered getCellSignalStrengths

Add a version of getCellSignalStrengths that allows
the caller to pass in a particular class version of
interest, which returns only valid matching signal
strengths of that type. A caller may now do:

for(CellSignalStrengthLte lte : ss.getCellSignalStrengths(
        CellSignalStrengthLte.class)) {
    return lte.getRssnr();
}

Bug: 123939399
Test: (cts) atest SignalStrengthTest
Change-Id: Ia64d25df4e782f76e6474b92ca7cd60c2f7d2f5e