Add ACTION_DESKTOP_MODE_EXIT_MODE_ON_LAST_WINDOW_CLOSE latency.

This measures the delay from when the user closes or minimizes
the last open window in desktop mode until the animation to exit
desktop mode draws its first frame.

Flag: EXEMPT metric change

Bug: 390544490
Test: Run `statsd_testdrive 306` and ensure that latency metric is
      reported.
Change-Id: I6e9f5c248c57d6094ad6472506d93c9eefba5cb4