Fix application font loading in basic font db

Fix application font loading in basic font db

We need to reregister fonts in initializeDb because basic font db
doesn't have an internal record like fontconfig does, so just
repopulating the font database won't work. db->reregisterAppFonts
is now used properly as intended (reregister application fonts
after the system font database has been cleared).

Also, static variable 'initialized' in initializeDb() is removed
since we check privateDb()->count to see if it needs to be populated
again.

Task-number: QTBUG-22063

Change-Id: Ifc66392b56b72acbe08b99256c61421c204be5d7
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>