applets/systemtray: Do not open context menu on mouse pressed for SNI

The prefered is to show context menu on mouse pressed event, not click,
but this causes few issues for SNI.

Some legacy applications still relies on XEmbed to show tray icon.
We use xembedsniproxy to relay the mouse press signals to the XEmbed client
but some applications (including GTK) validate the mouse state.
Mouse released state is expected, but as it is still pressed context
menu does not show.

BUG: 409768
FIXED-IN: 5.24.3


(cherry picked from commit a02d1ca2)