bug(android): `getAssociatedLexicalModel()` always returns first model for language

If there are multiple models installed for a language (e.g. for Khmer), `getAssociatedLexicalModel()` will always return the first model. There is no indication if there are more than one model installed for the language.

A similar problem exists for `registerAssociatedLexicalModel()`.

Documentation: [`getAssociatedLexicalModel.md`](https://github.com/keymanapp/keyman/blob/master/android/docs/engine/KMManager/getAssociatedLexicalModel.md)

Start with android/docs/engine/KMManager/getAssociatedLexicalModel.md and trace the implementations of getAssociatedLexicalModel() and registerAssociatedLexicalModel(). Reproduce the behavior with multiple installed models for one language, then determine and verify the intended handling for both methods without silently selecting the first model.

Written by the indexing model from the issue text.