Handle backup of AM itself specially

~~1.~~ Create a backup of AM itself as APK (with data) in Backup Folder.
As it suggests, AM would backup AM as an apk with a data file (readable only by the rightful owner, i.e encrypted) ~~when the user selects AM itself in batch (or single-ly),~~ whenever the user backups anything (https://github.com/MuntashirAkon/AppManager/issues/30#issuecomment-956037517).

On the first launch after installation of AM, AM would show a prompt if the user wants to import the settings from the backup folder (from all possible volumes).

~~2. Smart Keystore
Enabling it, AM would check if the device is suitable for Keystore apps to restore, if yes, AM would consider it as a regular app while backing up; if not while backing up, AM should only backup apk of the apps, regardless of what component is selected as other components are useless backing up anyway. While Batch (or single) restoring it'd simply install the apk (As/Till there is any known working method to hack Keystore) (Should help to save some space while batch backup by not collecting useless data).~~