MediaPlayerService: avoid invalid static cast

Bug: 30204103
Change-Id: Ie0dd3568a375f1e9fed8615ad3d85184bcc99028
(cherry picked from commit ee0a0e39acdcf8f97e0d6945c31ff36a06a36e9d)

void MediaPlayerService::AudioOutput::setVolume(float left, float right)
{
    ALOGV("setVolume(%f, %f)", left, right);
 Mutex::Autolock lock(mLock);
    mLeftVolume = left;
    mRightVolume = right;
 if (mTrack != 0) {
        mTrack->setVolume(left, right);
 }
}

void MediaPlayerService::AudioOutput::setVolume(float left, float right)
{
    ALOGV("setVolume(%f, %f)", left, right);
 Mutex::Autolock lock(mLock);
    mLeftVolume = left;
    mRightVolume = right;
 if (mTrack != 0) {
        mTrack->setVolume(left, right);
 }
}
