Make get/setSystemThresholdReportingRequestedWhileIdle @SystemApi

SignalStrengthUpdateRequest#setSystemThresholdReportingRequestedWhileIdle
is built to replace the deprecated feature
PhoneStateListener#LISTEN_ALWAYS_REPORTED_SIGNAL_STRENGTH.

The only client (Bluetooth) does not allow hidden API call.
@SystemApi is the next option while the API is not intended
to be public yet.

The javadoc is also updated to be more precise on when the
permission is required.

Bug: 205544105
Test: atest TelephonyManagerTest
CTS-Coverage-Bug: 205602896
Change-Id: Ia72cc1c77cf5b97458f8f6191a24bfa77a861234