Feature/november 2021 changes (#790)
* Fixed issue with full url of subtitle for HLS data source
* Kotlin migration (#730)
* Rename .java to .kt
* Kotlin migration
* Kotlin migration
* Rename .java to .kt
* Kotlin migration
* Rename .java to .kt
* Kotlin migration
* Kotlin migration
* Kotlin migration
* Kotlin migration
* Kotlin migration
* Kotlin migration
* Updated ExoPlayer version to 2.15.1
* Fixed crash related to Android 12
* Added missing bugfix: Fixed issue with full url of subtitle for HLS data source.
* Fixing Android 12 issues
* * Fixed install page from docs.
* Fixed one of the showcase images
* * [BREAKING_CHANGE] Split controlsHidden into controlsHiddenStart and controlsHiddenEnd.
* * Added onTapDown handle for material and cupertino progress bar to handle show and hide of controls.
* General refactor
* * [BREAKING_CHANGE] Added to Function(bool) onPlayerVisibilityChanged to customControlsBuilder in [BetterPlayerConfiguration]
* Format
* General refactor
* Updated screenshots
* Updated versions
* General refactor
* * Fixed video in list example
