playlist_viewer: add character-based Now Playing indicator
Addresses issue where it became difficult to find the
currently playing track once you scrolled away from it,
either with icons disabled in the Playlist Viewer, or if
the theme's icon set used whitespace for the Music icon.
An example scenario would be when you're moving some track
and are trying to put it next to the currently playing one.
Change-Id: I073a7e55fb723eeff755e32a78f88dcc7db1c245
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