TextClassificationSessionId.flattenToString() public
TextClassificationSessionId.flattenToString() is public, so that
text classifiers can log the session ID as a string.
Also, fix toString() method in TextClassifierEvent.
Bug: 124437773
Test: Added CTS for flattenToString().
Test: Checked toString output.
Change-Id: If615703a0bdb3762b05da504b3883f85daf17afe