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
diff --git a/cmds/statsd/src/atoms.proto b/cmds/statsd/src/atoms.proto
index 790a6d1..d0596c4 100644
--- a/cmds/statsd/src/atoms.proto
+++ b/cmds/statsd/src/atoms.proto
@@ -1264,6 +1264,9 @@
     }
     optional State state = 1;
     optional string id = 2;
+    // Last active session in ms.
+    // 0 when the port is in connected state.
+    optional int64 last_connect_duration_millis = 3;
 }
 
 /**