[Autofill] Remove parameter from OnAfterTextFieldValueChanged()

This CL removes a `std::u16string text_value` parameter from
AutofillManager::Observer::OnAfterTextFieldValueChanged().

That parameter had been introduced when FormFieldData::value() was
ambiguous.

As a side-effect, this CL refactors some Compose tests, which previously
called the AM::Observer event handler directly. After this CL, the
tests trigger the events via AutofillManager.

Bug: 40227496
Change-Id: I1ed4afe28756234b5e81cc2b55e1e4f1a12e578f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7581936
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1585526}