Refactor RouterAdvertisementDaemon with FdEventsReader

This patch changes the implementation of UnicastResponder in the
RouterAdvertisementDaemon with FdEventsReader. The change includes
replacing Thread with HandlerThread, create socket in non-blocking
mode (required by FdEventsReader) and use FdEventsReader to listen
to the solicited RS packets. Two commands (CMD_START & CMD_STOP)
were added to control the lifecycle of the FdEventsReader.

Bug: 246928127
Test: atest TetheringPrivilegedTests TetheringIntegrationTests \
      TetheringTests
Test: Enable WiFi hotspot and connected with another Pixel client:
      1. Tcpdump on the Pixel client, checking the RA packets
      2. Verify IPv6 addresses were configured
      3. ping6 2001:4860:4860::64
Change-Id: I6e89903df90864e3bb34d498e353d6f87193e9d6