docs: Android KMManager TODOs for example usage

[darcywong00]

          Approving but I think there are future TODOs. Ideally, the examples will include surrounding code to help clarify when and where a function may be called

Originally posted by @mcdurdin in #1294 (review)

Applies to:

• KMKeyboard/index
• KMManager/executeHardwareKeystroke
• KMManager/getKMKeyboard
• KMManager/getLexicalModelsList