[video_player] ExoPlayer 2.9.1, Android 9, Demo fixes

[ened]

This upgrades the video_player dependencies on Android to the latest ExoPlayer version, 2.9.1.

Along with that, security requirements for Android 9 have been applied.

ExoPlayer 2.9.1 is now shipped in Java 8 compatible byte code. This is fully supported on modern Android toolchains, however - users will need to upgrade their build.gradle configuration to include the following section, else their builds will break:

android {
    compileOptions {
        sourceCompatibility 1.8
        targetCompatibility 1.8
    }
}

I consider this change inevitable and flutter should always use the latest build environments.
We might want to document it somewhere, though. Also, the plugin / project templates will need a update.

[dnfield]

Thanks for doing this!

A few minor adjustments/requests, but I think this is a good idea overall!

[ened]

@dnfield Thank you for reviewing.

Here are the details rgd. the Java 8 language change:
https://developer.android.com/studio/write/java8-support

[dnfield]

I think the Java 8 stuff should be fine, we're not using Jack or retrolambda.

If you want to update any of the syntax for Java 8 that should be fine, as long as we don't end up using any language features that break earlier version support.

This LGTM, thanks for the contribution!