Refactoring and some fixes around TextClassifierService

This change is mainly to address the comment
"TextClassifierService.onDetectLanguage receives a callback but
immediately calls callback.onSuccess before method returns, that is
unexpected behavior. Method should either have a return value or post
callback. Same for onSuggestConversationActions" from API  council.

1. Like other services that allows developers to extend,
   APIs in TextClassifierService are now delivered in the main thread.

2. The default implementation of onDetectLanguage and
   onSuggestConversationActions now run local textclassifier in a worker
   thread. So it won't block the main thread and make sure the callback
   is called after the function returns.

3. Refactor the code so that fewer boilerplate codes around the callback.
   We are now using ITextClassifierCallback to replace all the existing
   I*Callback. This allows us to write template code easily.

BUG: 120841922

Test: Enable system textclassifier tests and run atest TextClassifierTest.java
Test: Manual. Try out smart selection and related features.

Change-Id: I33b2cc80f968fb8b5d5007a153b6e50041e57ce6