Rename SignalThresholdInfo.Builder#setThresholdUnlimited

Use method overload instead of two distinguish names since they
are both used to set thresholds but for different callers.

Bug: 177956167
Test: atest com.android.internal.telephony.SignalThresholdInfoTest
Change-Id: I56cba06f8d70d87aa188c54d58fefa6d785f5054