Drop AsyncTask, use AndroidX

Task/android 11 fixes (#13)

* Update build scripts for API 30 and AndroidX

* Update code to more modern standards, check for missing external cache

- Set language level to Java 8
- Use AndroidX
- Use RxJava instead of deprecated AsyncTask
- Raise minSdkVersion to 21
- Remove unneccessary WRITE_EXTERNAL_STORAGE permission
- If external cache is unavailable, fall back to internal
- Fix typo in filename filter, so we actually clean up

* Bump version number

* Accept license for android tools

* Try again to accept license for android tools

* Fix typo

* PR Feedback