Modify the class modifier to make it more friendly to other image lib…

[Haoxiqiang]

I'm writing a wrapper for glide and I don't want to use the original source for a project.
for android, the following two APIs are required.

Apng.Companion.isApng(source)
ApngDrawable.decode(xxx)

[CLAassistant]

All committers have signed the CLA.

[r-ralph]

Thank you for creating a PR and sorry for late response.
Since the Apng and ApngDecoderJni classes can modify internal data/state without regard to current state, there are no plans to make these classes public to protect data integrity.
It is possible to work with the current public API only, even when used with Glide.