Improve design of playback UI.

Change background color of playback UI.
This patch also uses built-in music controller view instead
controls.xml to easily control player behavior.
Add a field 'filePath' in Song Class to load music files.

Currently, there's no way to load music files as we don't know
row path of music files. This patch resolves each music's file path from
android media store and save them to BPM table.
From now, <Song> class holds file path so that we can easily get it from
each music file.