Add swipe to auto-brightness at 0% in exoPlayer

[stm91]

Changes

• enhances PlayerGestureHelper to allow swiping past 0% brightness to restore auto brightness from system
• adds "A" ui-indicator to brightness overlay when auto-brightness is enabled

Issues

• adds feature mentioned in comment of Volume and Brightness Controls cover content #1185

[Maxr1998]

Hi, thanks for your contribution. We're in the process of migrating the player to Jetpack Compose (see the compose-player branch for details), so it unfortunately doesn't make sense to merge your PR. However, I like the concept behind it and will incorporate the feature in the rewritten player later on.