Fix leak of DhcpServer on tethering stop error

Tactical, minimal fix to make sure the DhcpServer is stopped when
disabling tethering. The current code may throw if the interface is gone
when calling mNMService.setInterfaceConfig() on teardown. Make sure DHCP
is stopped regardless of what happens.

A longer term fix will be to figure out the right flow to teardown the
interface and avoid errors on setInterfaceConfig().

Bug: 124520692
Test: Manual: no more socket leaked when stopping USB tethering
Change-Id: Ib87d89a2e5fdb64b8f3dfae48d82c970f3ae6161