Refactor model listing / selection code to support other types of model

Currently, listModelFiles and findBestModelFile methods only support annotator model.
But we want to extend them to support other models as well, like langID and actions.

Thus, introducing ModelFileManager, which provides listModelFiles and
findBestModelFile. ModelFileManager takes a Supplier<List<ModelFile>> to list model files.
For different types of model, we just need to provide a different supplier to the ModelFileManager.

There should be no behavior change.

Test: atest frameworks/base/core/tests/coretests/src/android/view/textclassifier/ModelFileManagerTest.java
Test: atest frameworks/base/core/tests/coretests/src/android/view/textclassifier/TextClassificationManagerTest.java

Change-Id: I4fc3fd1c9246383ee5d906792bb14b96dbf0a79f