Add a specialized implementation for retrieving apps installed via Flatpak

This change:

1. Fixes a bug where the same app could be reported twice

   Every app installed via flatpak has 2 desktop files under its directory, take com.google.Chrome as an example:

$ find . -name "com.google.Chrome.desktop"
./x86_64/stable/ca5617c1195bd48bd3cfae549ea738f5bfc51f78aec1e7b025948339140b6abf/export/share/applications/com.google.Chrome.desktop
./x86_64/stable/ca5617c1195bd48bd3cfae549ea738f5bfc51f78aec1e7b025948339140b6abf/files/share/applications/com.google.Chrome.desktop

So our generic implementation will treat these 2 desktop files as 2 apps.

2. Improves the performance of the get_all_apps() interface