Fixes avplayer seek and state on iOS (not tested on OSX)

The previous implementation was using playAtTime_, but 100% sure it was
not working as playAtTime_ schedules the audio to play at a specific
time, not to seek within the current audio file. To seek to a specific
position in the audio, we should use the currentTime property of
AVAudioPlayer.

In addition, i added a delegate to correctly catch when the sound is
stopping when reaching eof, which make the state property correctly
work.

kivy/core/audio/audio_avplayer.py

@@ -6,7 +6,7 @@
 __all__ = ('SoundAvplayer', )
 
 from kivy.core.audio import Sound, SoundLoader
-from pyobjus import autoclass
+from pyobjus import autoclass, protocol
 from pyobjus.dylib_manager import load_framework, INCLUDE
 
 load_framework(INCLUDE.AVFoundation)
@@ -40,19 +40,25 @@ def unload(self):
     def play(self):
         if not self._avplayer:
             return
+        self._avplayer.delegate = self
         self._avplayer.play()
         super(SoundAvplayer, self).play()
 
     def stop(self):
         if not self._avplayer:
             return
+        self._avplayer.delegate = None
         self._avplayer.stop()
         super(SoundAvplayer, self).stop()
 
     def seek(self, position):
         if not self._avplayer:
             return
-        self._avplayer.playAtTime_(float(position))
+        avplayer = self._avplayer
+        avplayer.stop()
+        avplayer.currentTime = float(position)
+        if self.state == 'play':
+            avplayer.play()
 
     def get_pos(self):
         if self._avplayer:
@@ -68,5 +74,9 @@ def _get_length(self):
             return self._avplayer.duration
         return super(SoundAvplayer, self)._get_length()
 
+    @protocol("AVAudioPlayerDelegate")
+    def audioPlayerDidFinishPlaying_successfully_(self, player, flag):
+        self.stop()
+
 
 SoundLoader.register(SoundAvplayer)