TextClassification.text is nullable

Removes the @NonNull requirement on TextClassification.text.
e.g. TextClassifier.NO_OP doesn't classify text and thus returns
TextClassification.EMPTY which doesn't set TextClassification.text.
This is ideal as we need not make copies of this object when NO_OP
was performed.

Test: bit CtsViewTestCases:android.view.textclassifier.cts.TextClassificationManagerTest
bit FrameworksCoreTests:android.widget.TextViewActivityTest
bit CtsWidgetTestCases:android.widget.cts.TextViewTest
Change-Id: I0b3ac5bf0027cbd67ba40d1110f1100280c8c3a2