Cleanup TextClassifier APIs

Major changes:
1. ConversationAction is now in top-level.
2. Removed TypeConfig, and repurposed TextClassifier.EntityConfig for
   general use. It would be better to rename it to be something like
   TypeConfig. But just a bad name is probably not worth to deprecate
   the existing APIs.
3. Hints constants are moved to Request object.
4. Action constants are moved to ConversationAction object.

Test: atest TextClassifierTest.java

BUG: 120841922

Change-Id: Ia466aaf4c5050a9c7e404dcd3b295f5ef7e4ce6f