Don't show timeline tooltips outside of the preview selection

As of #3840, if there is a preview selection and you click the activity graph outside of that selection, we now ignore that click to some extent: The click only resets the preview selection. It no longer switches the selected track or selects the call node of the clicked sample. You need a second click to do that.

However, we still display the activity graph tooltip if you hover the graph outside of the preview selection. This is misleading: It sends the message that "You can click here to see the stack in the tooltip". But that's not true; clicking doesn't do anything other than resetting the selection.

So I think we should only display the tooltip while the mouse is inside the preview selection. If the mouse is outside the preview selection, no tooltip should be displayed. Once a click happens at that position and the selection is reset, the tooltip should appear immediately.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-511)