Introduce TextSelection.Builder.setIncludeTextClassification and...

Introduce TextSelection.Builder.setIncludeTextClassification and TextSelection.getTextClassification()

This small API change enables us to optimize the latnecy of smart
selection.

To enable smart selection, TextView/WebView always call
textClassifier.suggestSelection and textClassifier.classifyText back to
back. There are a lot of overlaps between suggestSelection and
classifyText, e.g. both API calls classify the entity type of the text.
This optimization is important to make smart selection working
responsively, especially that we enforce a 200ms timeout in TextView.

This small API change allows the TextClassifier to implement smart
selection with just one single API call.

This change is backward-compatible.
If the text classifier is not updated to return the text classification
result in suggestSelection yet, clients can learn that by checking if
textSelection.getTextClassification() returns null. If so, the client
can then fallback to call ClassifyText().

You may find the textview integration in ag/13092679

Design doc: go/android-s-suggestselection

Bug: 173512834

Test: atest TextSelectionTest
Change-Id: If4cc827aa19d6d14c885cf10cd5ba16aa3041cf6