Fix crash and duplicated ethernet tethering request

This change fix two things:
1. Handle ethernet callback in internal thread to avoid crash. IpServer
should be created from tethering thread, otherwise mIpNeighborMonitor of
IpServer would throw
   IllegalStateException("start() called from off-thread")
2. Ethernet tethering request may be duplicated if multiple
startTethering is called but no stopTethering

Bug: 130840861
Bug: 148824036
Test: ON/OFF ethernet tehtering manually
      atest TetheringTests

Change-Id: I7c5127e96d80d077735010d2e62c7227805ccb10