Add ACTION_DESKTOP_MODE_ENTER_APP_HANDLE_DRAG latency.

This measures the delay between steps 2 and 3 in the following
sequence:
1) Drag an app's handle while it's in full screen mode towards
   the middle of the screen to enter desktop mode.
2) Release the drag.
3) Window's bounds start animating to their new position, and the
   device enters desktop mode.

In a past bug, this delay was large. It has since been fixed to
be acceptable, but this latency is being tracked to catch any
other cases that may be missed and potential future regressoins.

Flag: EXEMPT metric change
Bug: 381408940
Test: Put tangorpro in HSUM mode:
      `adb shell cmd user set-system-user-mode-emulation headless`
      Run `statsd_testdrive 306` and ensure that latency metric is
      reported. This was done with
      `enable_desktop_wallpaper_activity_for_system_user` enabled and
      disabled. The latency was close to 1 second with that flag
      disabled and ~100 milliseconds with that flag enabled. This is
      expected.
      atest DragToDesktopTransitionHandlerTest DesktopTasksControllerTest
Change-Id: Ia523734ce44588a52166da7bec979bf7e049d3e3