[video_player] Add speed adjustment support for Android

[sfanxiang]

Android implementation for flutter/flutter#16624.

[ened]

@sfanxiang nice PR! Could you work on the formatting issues that prevent the PR from going green?

[sfanxiang]

Turns out fd8ad8a did the formatting, and 3f8abcf bumped ExoPlayer to 2.9.1, so I rebased this patch.

Now CI complains:

> Could not find com.google.android.exoplayer:exoplayer-core:2.9.1.

[amirh]

Note for reviewer: #1400 seems to be doing the same.

[cbenhagen]

@sfanxiang thank you for your contribution! Unfortunately the structure of the plugin has changed lately to support implementations on more platforms. Mainly the plugin was moved to a new subdirectory and now uses a common platform interface. Adapting your PR to the new structure should be straight forward. If you have questions please ask!

Please also make sure to include tests for the new functionality. It would also be great to add this to the iOS and web parts of the plugin.

[cbenhagen]

Actually I think this could be closed in favor of #1400 which also includes the iOS part.

[sfanxiang]

@cbenhagen Feel free to close this if you believe #1400 is more helpful.

[cbenhagen]

I can not close it. But if you think your Android implementation is better then the one in #1400, you could also comment there to improve it. I know your PR was first. But in the end the only thing that counts is to have the best implementation with the most complete tests and documentation no?

[ditman]

Published video_player v0.11.0, which contains the playback speed feature!