Make APNGDecoder#getLoopCount public method

[phicdy]

New Issue Checklist

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Issue Info

Info	Value
Device Info	e.g. Huawei Mate 30 pro
System Version	e.g. 6.0
APNG4Android Library Version	e.g. 1.0.0
Repro rate	e.g. all the time (100%) / sometimes x% / only once
Repro with our demo project	e.g. does it happen with our demo project?
Demo project link	e.g. link to a demo project that highlights the issue

Issue Description and Steps

Hello @penfeizhou , thank you for awesome library. In some cases, I'd like to get loop count, but is not opened. Can we make it public method ? If there are not any concerns, I'll create PR. https://github.com/penfeizhou/APNG4Android/blob/0d3c268869ed391995f360c7264dea86966a34de/apng/src/main/java/com/github/penfeizhou/animation/apng/decode/APNGDecoder.java#L66-L69