Refactor ui_events_helper and move into common/input.

This CL breaks ui_events_helper into common input code (renamed
events_helper) and 'synthetic_gesture_target_aura' specific code in
browser and moves it into common input code. This change is required to
help moving touch_emulator which is required to move
RenderWidgetTargeter & RenderWidgetHostInputEventRouter into
common/input.

Doc Link:
https://docs.google.com/document/d/1dgbvnRChsvfMpXceQ24UbTdFwJpmk5VIl7WXHwExAC4/edit?tab=t.0#heading=h.7wb78gd5w8i0

Bug: b:330555923
Change-Id: I1a2ad251ea366fdf47ea457f59a9aa3c9cd83ade
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5522373
Commit-Queue: Aman Verma <amanvr@google.com>
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1304731}