Add log for dismissal when new screenshot is taken

Currently, we log when the screenshot UI is dismissed due to timeout,
tapping the 'X' button, tapping the preview, or tapping any of the
action buttons. However, there is an additional way to cause the UI
to be dismissed, which is to take a new screenshot while the UI is
up (in which case we dismiss the old UI immediately). We should log
this to get a better picture of how people use/interact with the
screenshot UI.

This change adds the log (previously registered using the uievent
cli) and logs when a new screenshot is taken, if there's UI on the
screen and we aren't already dismissing for some other reason (to
avoid duplicate logs).

Bug: 166249049
Test: manual
Change-Id: I4a1d7cf8c1309a3198593eef004a2b18f94f809f