Minor Content Capture-related changes.

- Refactored onProvideStructureForAssistOrAutofillOrViewCapture() into a common,
  hidden onProvideStructure(viewFor) method on View that (hopefully :-) makes
  it easier to understand / extend.
- Renamed / documentted some methods on TextView.

Bug: 111276913
Test: atest CtsAutoFillServiceTestCases CtsAssistTestCases \
            AssistFrameworksCoreTests:AssistStructureTest

Change-Id: I328ce3f26a42e7408015cc0014a3f707801ebeb9