Preparation to expose SignalThresholdInfo as public class

Introduce RAN to better associate it with thresholds. Later CL
should be able to further simplify APIs which ask for both RAN and
SignalThresholdInfo.

Validate threshold values during constructing.

Deliberately hide all the public fields and methods, so the following
CL will only need to expose designed properties. Make it easy
for API review.

Bug: 164432835
Test: atest com.android.internal.telephony.SignalThresholdInfoTest
Change-Id: Ib0600c43b90ab0231b29f989976cb090f4b37bf8