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
Merged-In: I03c4161a01861b47cf878945f7f5cc77c821cd57
Change-Id: I03c4161a01861b47cf878945f7f5cc77c821cd57