Fix the potential double transition in DhcpState#enter

The double transition problem might happen when interface is gone by
accident after starting DhcpClient. The StateMachine doesn’t allow
to call transitionTo() in enter().
Use deferMessage() instead to avoid this crash.

Bug: 145182281
Test: manual
Change-Id: I03c4161a01861b47cf878945f7f5cc77c821cd57
1 file changed