Refresh project + new features

[natario1]

First of all, great job @ypresto with this library, it is extremely useful. More than a PR this is a question. Is this project evolving in some direction?

I have forked the repo and spent a few hours today, doing some basic maintenance. At the time of writing this PR includes:

• refreshing dependencies
• updating gradle and target SDK
• moving to AndroidX
• adding a Logger class so we don't pollute the logcat
• fix the audio format bug as in Fix audio format validation #70
• adding Nullable and NonNull annotations here and there to support Kotlin
• lib now accepts files without an audio track (using a NoOpTrackTranscoder in this case) instead of throwing
• lib now supports content:// uris as input, instead of file descriptors. Avoids the need of writing a temp file. Should be used in demo app.

I plan to work a lot more on this in the next weeks and add new features. Want to do something about input aspect ratio (cropping using OpenGL might be a good solution), and perform actual frame dropping (simply setting the frame rate to the output format does not mean that MediaMuxer is going to do it, AFAIK).

I am happy to contribute to the original repo but there's a lot to be done and speed is important to me at the moment. So if you are open to review / discussions / merging, let's do it! If not, feel free to close this PR and I will work on my fork. As said, I hope to be actively working on the features that I think are missing, and can help you maintain the repo if you want.

Either way, this is a great lib, thank you!