Added @Nullable and @NonNull annotations for Kotlin use

[sfeatherstone]

This library already has a strong notion of when parameters should or should not be null. This formalises this contract in a way that Kotlin recognises.

• I have suppressed warnings now generated by adding the annotations. In all cases, I have checked the code flow to ensure that the warning suppression is correct.

Note: ImageDecoder.decode() The context is not marked as @NotNull because files do not require a context to be set. All other URI type would result in a NPE

Formatting follows exist code and Google java style guide.

[davemorrissey]

Nice, thanks! I'll try this out in the kotlin sample.