Bug fixes and support for API < 22

• Fixes a crash during getExternalFilesDir
• Fixes build.gradle for Android Studio
• Fixes some wrongly named folders
• Adding a fixed version for okhttp (3.12.+). This should ensure support for devices with API < 22

Sumup of changes since the fork

• Integration of Windows implementation
• Support for Android 10 & 11 (scoped storage; see ReadMe for more information)
• Better promise handling for opening files on Android (Rejecting the promise if no app is available for opening a file)
• Renaming the project
• Updating the ReadMe