seek, get_pos and get_length support for mediaplayer

[akshayaurora]

Testing with the following code and android_mediaplayer_seek_length branch of kivy for the changes that were needed to audio_pygame. Currently wrong pos(off by a few secs) is displayed with vorbis files (known issue with Android MediaPlayer).

from kivy.app import App
from kivy.core.audio import SoundLoader
from kivy.clock import Clock
from kivy.logger import Logger
from kivy.lang import Builder

root = Builder.load_string('''
BoxLayout:
    audio_pos: 0
    orientation: 'vertical'
    Label:
        text: str(slider.value)
    Slider:
        id: slider
        min: 0.0
        value: root.audio_pos
    BoxLayout
        Button:
            text: "play"
            on_press:
                root.sound.play()
                root.upd_pos()
        Button:
            text: 'stop'
            on_release:
                root.sound.stop()
                slider.value = 0.0
                root.upd_pos(stop=True)
        Button:
            text: 'seek to 20.0'
            on_release: root.sound.seek(20.0)
''')

class TestApp(App):
    def build(self):
        root.sound = SoundLoader.load('audio02.ogg')
        root.upd_pos = self.upd_pos
        return root

    def upd_pos(self, stop=False):
        if stop:
            Clock.unschedule(self.get_pos)
        else:
            Clock.unschedule(self.get_pos)
            Clock.schedule_interval(self.get_pos, 1./9.)

    def get_pos(self, dt):
        root.ids.slider.max = root.sound._get_length()
        root.audio_pos = root.sound.get_pos()

if __name__ == '__main__':
    TestApp().run()

[tito]

Dude, perfect PR. Really. !