Read/write application data on android
Reading and writing application data requires at least system permissions (user id 1000 or less) or explicit permissions (user id of the application itself). Some applications share user ids (like free and pro versions of an application) to share the same data but as a normal, not system user, you are unable to even list the files inside the /data partition where all the application data is stored.

The backup procedure from android/google itself is done with system permissions as the system user (id 1000) and does only work on applications which explicitly set the allowBackup tag to true.