[Compose] Use OnAfterCaretMovedInFormField to extend delay

This CL does three things:

First, it modifies the On[Before|After]CaretMovedInFormField methods
of AutofilManager::Observer to pass the selected text.  The selected
text is empty if there is no selection.

Second, it uses OnAfterCaretMovedInFormField in the
PractiveNudgeTracker to extend the delay instead of
OnTextFieldDidChange.  Caret movement is a super set of the text field
change event and covers more of the cases such as cursor movement with
the arrow keys.

Third, it updates the chrome_compose_client tests to use mocktime to
better test the nudge delay.

Bug: b/335082750
Change-Id: I519ae2c1be701163f36b48e57da0f5b2896cb47d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5606348
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Reviewed-by: Anthony Cui <cuianthony@chromium.org>
Commit-Queue: Trevor Perrier <perrier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312403}