Refactor PluginManager: store plugins in private files dir

[itsyourap]

This pull request updates the plugin storage directory to the app's private files directory, ensuring a clean state before loading plugins, and improving error handling when the plugins folder does not exist.

Plugin directory management improvements:

• Changed the plugin storage directory from the external files directory to the app's private files directory (context.filesDir) to avoid SecurityException when loading dex files.
• Cleaned up all non-existing plugins in the plugins directory by comparing them with the local plugins directory before loading new plugins to prevent stale plugins from persisting.
• If local plugin folder (in Internal Storage) does not exist, then do not create it and remove all local plugins currently installed.

Error handling and plugin loading logic:

• Improved handling when the local plugins folder does not exist by logging a message and marking plugins as loaded, instead of attempting to create the directory and logging a warning.
• Ensured that all local plugins are fully refreshed by removing the local plugins key before loading.

Related Issues

• Fixes #2076