Add app start source info from notification

This provides the time from action up time in SystemUI
to the time system received the activity intent.

Bug: 166614700
Test: Enable statsd log: "adb shell cmd stats print-logs"
      Click notification to launch activity and check logcat.
      adb logcat | grep statsd | grep "(48)"
      The line should contain 0x100000->2[I].
Change-Id: Icd43f38ebc8201156844adcc0cba922212185ea6