⚗🐛 [RUMF-1293] discard dead clicks when activity occurs on pointerdown

[BenoitZugmeyer]

This ticket is part of an effort to improve dead clicks.

• ⚗🐛 reset input state at the beginning of each click #1968 will fix an issue where a lot of dead clicks were completely ignored
• ⚗🐛 [RUMF-1296] use pointerup to trigger click actions  #1958 will discard dead clicks when something happens during the 'pointerup' event (previously only things happening during the 'click' event were taken into account)
• ⚗🐛 [RUMF-1293] discard dead clicks when activity occurs on pointerdown #1979 (this issue) will discard dead clicks when something happens during the 'pointerdown' event

All three fixes are behind the dead_click_fixes and will be released together after some dogfooding.

Motivation

We observed that in some cases, click actions are flagged as "dead click" when the UI updates during "pointerdown" or "mousedown" events (happening before the "click" event).

Changes

With this PR, we'll observe page activity occuring during the "pointerdown" event, and will discard the "dead click" frustration if some activity occurs.

Testing

• Local
• Staging
• Unit
• End to end

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I have gone over the contributing documentation.