Add sound for bootanimation

With the bootanimation starting, a mp3 is played. The mp3 file is
contained in the bootanimation.zip. Its name is audio.mp3.
The sound volume is a percent value of the max STREAM_SYSTEM volume
that comes from a persistent system property. The system property is
set by a java module according to the global sound setting.
The default volume is 73 (73% of the max STREAM_SYSTEM volume)
that is the same as the STREAM_SYSTEM default volume.
0 indicates the target is in silent or silent+vibrate mode.

Change-Id: Ic798e7ae6e9415d544d4a68f4315d34e259c8fb8