TextClassificationManager API updates.

- Changes getDefaultTextClassifier to getTextClassifier
- Adds a setTextClassifier API

Test: cts-tradefed run cts-dev -m CtsWidgetTestCases -t
android.widget.cts.TextViewTest
Bug: 35763797

Change-Id: I24e86272cf58bf6864505bee3b06978c1c0d46e5