Introduce SignalStrengthUpdateRequest to customize signal update request

Privileged apps can send the request through TelephonyManager APIs
(in following CL) to monitor signal strength change with interested
RAN/SignalMeausrementType/Thresholds.

Bug: 164432835
Test: atest com.android.internal.telephony.SignalStrengthUpdateRequestTest
Change-Id: Ifba384108b6627e4b808be34e6d828c93e8df995