Limit the uniqueness of SignalMeasurementType instead of RAN in

SignalStrengthUpdateRequest

Support different SignalMeasurementType with same type of RAN
is valid user case. For example, client may be interested to
measure both RSRP and RSRQ for EUTRAN.

Throw IAE if the SignalThresholdInfo collection is empty. This
is behavior specified in the public interface.

Bug: 198318496
Test: atest SignalStrengthUpdateRequestTest
Change-Id: I87ea5e356189f61fc58dff80456aff35ea2a8e55