IpConn metrics: add dhcp transition times

This patch adds lifetime durations of DhcpClient states to
DhcpClientEvents.

To record the duration of a state, the event is now recorded when the
DhcpClient state machine exits that state.

In addition this patch removes event logging of StoppedState,
DhcpState and DhcpHaveLeaseState.

Change-Id: Ibd37b5e3070f35113b6b45942b1e1ff19c27a90b