Add app start source information to statsd

Add more information about how the activity is launched
with the event time which is closer to user interaction.

Bug: 166614700
Test: ActivityMetricsLaunchObserverTests#testOnReportFullyDrawn
Test: Enable statsd log: "adb shell cmd stats print-logs"
      Check the logcat output contains the given string.

Change-Id: If153f0b9e8d8969a2122e5f2b8aac4a83aa524c9