Android: migration needed to get rid of metadata

Historically, the Android app uses metadata to keep some media related data that were not implemented in the ML.

Now is the time to get rid of it for harmonization.

There are two metadata to get rid of:

• IMedia::MetadataType::Seen has been used as the single source of truth (see vlc-android#2030 (closed)). IMedia::playCount() is never used anywhere in the app and has been corrupted by a misuse. The app indeed sets the progress to 1.0 at the start of the playback, making it marked as seen for an incomplete playback : https://code.videolan.org/videolan/vlc-android/-/blob/master/application/vlc-android/src/org/videolan/vlc/media/PlaylistManager.kt#L922

We have to migrate from the meta to the playCount to make it the new SSOT

• IMedia::MetadataType::Progress (now obsolete) is still currently used in the app to resume the playback, when IMedia::progress() is used in the UI. It seems that both data are valid and no migration is needed here

EDIT: IMedia::MetadataType::Progress is still useful in case we don't know the media length. We use the meta to be able to resume the playback when the saved data in the Media table is useless (duration: -1 and progress -1)