mpris: Hide notification when !CanPlay, instead of closing player

mpris: Hide notification when !CanPlay, instead of closing player

I have observed a client in the wild (Chromium again) set CanPlay to
false momentarily while it is loading the next song. Previously, the
code would close the player proxy in that case, meaning that after
playing one track, the MPRIS message would disappear and never come
back.

However, I think this use of CanPlay, while apparently not usual, is not
incorrect according to the spec. We should hide the message instead of
closing the player proxy.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1362