Add latency metrics for recents gesture

Pass the touch down time to RecentsAnimation#startRecentsActivity.

Bug: 169221287
Test: Enable statsd log: "adb shell cmd stats print-logs"
      Touch gesture navigation bar.
      adb logcat | grep statsd | grep "(48)"
      The line may contain 0x100000->4[I] 0x110000->20[I]
      that means 4=by recents and 20=latency 20ms.
Change-Id: I81ee804895b7712f4d925736f5b4694c11a12cbe