Storage refactor for EntityConfidence

Caching the sorted entity list so users don't need to be careful to cache
the result of getEntities (previously dont by TextSelection and
TextClassification, but not TextLink). Also switched to ArrayMap as it's
better suited for small maps like the ones generated by the classifier.

Test: Ran FrameworksCoreTests
Change-Id: I08cc9f72146ccab88b6a3624f3775a366c814f7a