Add TYPE_LINKS_GENERATED and make ...

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