Register NetworkRequest callbacks with a handler

Test: as follows
    - builds, flashes, boots
    - settings changes that cause the "x" to appear and disappear
      from the wifi icon don't crash
    - runtest frameworks-net passes (kinda no related)
Bug: 64133961
Bug: 77285360
Change-Id: I8af34d623ce265545a2ec42b56c5e87b4d077f58