Add ACTION_DESKTOP_MODE_EXIT_MODE latency.

This measures the delay from when the user provides input
to exit desktop mode until the animation to exit desktop mode
starts. "input" for this metric includes doing any of the
following to switch an app to full screen mode:
* Selecting the full screen button in an app handle's menu.
* Dragging an app window's handle to the top of the screen.
* Using a keyboard shortcut.

Flag: EXEMPT metric change
Bug: 390544490
Test: Run `statsd_testdrive 306` and ensure that latency metric is
      reported.
Change-Id: Iaa1c86a85e1a1d7b0f460a8b758ece16bdf9a0cb