Quit statemachine when dhcp server is stopped

This change fix the memory leak problem that dhcp server thread do not
shut down when it is stopped.

Bug: 161418295
Test: atest DhcpServerTest --iterations 100
      atest CtsTehteringTest
      manual on/off tethering and observed dhcp server thread

Change-Id: I58a880b110af99ba5fedc95490e6b2a3519e1c08
Merged-In: I58a880b110af99ba5fedc95490e6b2a3519e1c08