IpConn metrics: add times to DHCP bound state

This patch adds DhcpClientEvent logging for:
 - time from init state to bound state
 - time from bound state to bound state when renewing

Example:
ConnectivityMetricsEvent(12:04:33.353, 0, 0): DhcpClientEvent(wlan0, WaitBeforeStartState, 94ms)
ConnectivityMetricsEvent(12:04:35.772, 0, 0): DhcpClientEvent(wlan0, DhcpInitState, 2417ms)
ConnectivityMetricsEvent(12:04:35.787, 0, 0): DhcpClientEvent(wlan0, DhcpRequestingState, 15ms)
ConnectivityMetricsEvent(12:04:35.793, 0, 0): DhcpClientEvent(wlan0, ConfiguringInterfaceState, 6ms)
ConnectivityMetricsEvent(12:04:35.811, 0, 0): DhcpClientEvent(wlan0, InitialBoundState, 2457ms)
ConnectivityMetricsEvent(12:05:35.791, 0, 0): DhcpClientEvent(wlan0, DhcpBoundState, 59998ms)
ConnectivityMetricsEvent(12:05:35.988, 0, 0): DhcpClientEvent(wlan0, WaitBeforeRenewalState, 196ms)
ConnectivityMetricsEvent(12:05:36.003, 0, 0): DhcpClientEvent(wlan0, DhcpRenewingState, 15ms)
ConnectivityMetricsEvent(12:05:36.010, 0, 0): DhcpClientEvent(wlan0, RenewingBoundState, 219ms)

Change-Id: I2540dd102a6509dff55936e38015265659b20760