RouteDiscoveyPreference: Update javadoc

Added missing javadoc and updated some to clarify.
Renamed activeScan to performActiveScan.

This CL is to address issues in API review.

Bug: 149299503
Test: make api-stubs-docs-update-current-api
Change-Id: Ic8a630eb65daab190ec6e0a106c81aa6edfbf72d