Add API to support async network scans.

This CL includes the new Java API that supports async network scans.
Applications could get faster network scan results with this new API.
The long-term goal is to support Subsecond Switching which is separate
from what this CL is doing.

Bug: 30954762
Test: Unit test, run runtest -x NetworkScanRequestTest.java
Change-Id: I78cadaf838dee8439fe7417ef39f689990bf5705