Refactor SocketNetlinkMonitor to make it buildable with system sdk

NetlinkMonitor cannot be used when building with system sdk. This CL
refactored the SocketNetlinkMonitor into a separate class and use
factory method and droidstub to avoid referring NetlinkMonitor when
building with system sdk.

Test: m service-connectivity-mdns-standalone-build-test
Bug: 272392042
Change-Id: I2d97f6fe8a17febd155c81303d86cd558654ddf5
5 files changed