Fix erroneously sending RRs, pt1.
Firefox fires the blur event on both document and window. Chrome
only fires it on window, so on chrome we were not seeing the window
being un-focused. window seems to be the standard so just use that.
This isn't the end of the story though since wer can get mousemove
events without the window ever having gained focus, in which case
we'll continue to think the user is active for another 2 mins when
in fact all they did was pass their cursor over the window.
https://github.com/vector-im/riot-web/issues/9023
