Use event time of app side for latency metrics

There are 2 reasons not to use MotionEvent#getEventTime:
1. There is an existing TouchEventReported metric for touch latency.
2. In instrumentation test, the event time is set when creating
   the motion event for injection. It may cause the results to be
   5 times slower than the real case. That may lead to miss the
   cost from app to system.

Bug: 166614700
Test: Enable statsd log: "adb shell cmd stats print-logs"
      Touch gesture navigation bar or launch app from launcher.
      adb logcat | grep statsd | grep "(48)"
Change-Id: Ica5e7df685fe737bbc3ac60c3ec35bad15466726