Custom data location compatable with Android 11

[samnyan]

Goal: Custom data location works with Android 11 (API level 30)

Because of the new scoped storage of Android, apps can't access files outside app-specific directory directly.
Unity itself has no support for documents provider, so IO operation must handle by native code.

Todo:

• Android Native Plugin for required IO operation.
• Wrap required System.IO operation with UniversalIO class.
• Use native plugin to handle IO with "content://" uri in UniversalIO
• Compatable with Android 5.1 ~ 7.0 below (API level 24)