Log connection duration.

last_connect_duration_millis will be set to 0 for a connect event.
last_connect_duration_millis will be set to the amount of time the port
was left connected for a disconnect event.

Bug: 118783261
Test: Manually tested by running: adb shell cmd stats print-logs,
    adb logcat | grep statsd | grep \(70\)
Change-Id: Ifd1df55fb6317e789a2b04236ce0a6e4665e3664