Add horizontal padding to subtitle background

Please consider adding horizontal padding of (text) subtitle background. (As also previously discussed here: https://github.com/google/ExoPlayer/issues/7516#issuecomment-646066696)

This is how it currently looks like:

The style preview in system captioning manager does include horizontal background padding:

After replacing `BackgroundColorSpan` in `SubtitlePainter` ([with something like this](https://medium.com/@tokudu/android-adding-padding-to-backgroundcolorspan-179ab4fae187)):

At least for me personally, this change significantly improves the (at a glance) readability of subtitles on both small and large screens.