MediaScanner: check build fingerprint for system sound scan

Always scan notification, ringtones and alarm files under /system
  when the build fingerprint has changed since the last scan,
  because the dates on those files is not set correctly
  (by design), and hence cannot be trusted in the date-based
  scanning logic.
Remove some dead code

Bug 30476971

Change-Id: I638c787dc177f7f5fb17c1c2a576be190c1c85f9