Add a signature to TextSelection and TextClassification

A signature is a tag that a TextClassifier may use to identify an
object it returned for a given query. This is useful for logging
purposes.

This cl also removes:
 - TextClassification.getLogType() and related logging.
   This is already covered by selection event logging.
 - TextClassification.getVersionInfo(),
   TextSelection.getVersionInfo()/getSourceClassifier().
   These are now featured in the signature.

TODO: Write a container class that generates and parses signatures.

Bug: 69791269
Test: bit FrameworksCoreTests:android.view.textclassifier.TextClassificationManagerTest
Test: bit FrameworksCoreTests:android.widget.TextViewActivityTest
Test: bit CtsWidgetTestCases:android.widget.cts.TextViewTest
Change-Id: I294f7e4d16c98c6512d56d08d488b204c1f91d47