user_manager: Rework desktop file handling

* Update all desktop files on Android startup
* Remove non-existing applications on Android startup
* Preserve user comments and translations
* Preserve user modifications to Categories and Actions
* Preserve user modifications to NoDisplay
* Default system applications to NoDisplay=true

Closes: https://github.com/waydroid/waydroid/pull/339
Closes: https://github.com/waydroid/waydroid/issues/46