Added flag for background mode

[deshiknaves]

Added the missing implementation for play in background for exoplayer.

[wschurman]

@mattapperson - Friendly ping :)

[markusekblad]

@deshiknaves @mattapperson
Great stuff, I had one issue tho,
I tried this today and for me the

public void onHostResume() {
    if (playInBackground) {
        return;
    }
    ...
}

causes the player to start as paused even with paused false prop passed.

It also didn't play in the background. However, if I remove that check from onHostResume, and add it instead to

    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        if (playInBackground) {
            return;
        }
        stopPlayback();
    }

Then background audio works for me and there's no issue with the paused prop. I'm using a modal player and not the fullscreen one if that makes any difference.
(Tested on a Huawei Honor 8 and Samsung S6, both with Android 6.x)