Add the new RIL requests to start/stop network scan

This CL defines the RIL API which will be used to start and stop the
network scan. The long-term goal is to support Subsecond Switching which
is separate from what this CL is doing.

Test: Telephony sanity tests
Bug: 30954762
Change-Id: I71de3f1dc0790ca4b9a2b9a4827637cb337668e2
(cherry picked from commit ffcdbea8)