MediaPlayerService: avoid invalid static cast

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

sp<IOMX> MediaPlayerService::getOMX() {
    ALOGI("MediaPlayerService::getOMX");
 Mutex::Autolock autoLock(mLock);

 if (mOMX.get() == NULL) {
        mOMX = new OMX;
 }

 return mOMX;
}

sp<IOMX> MediaPlayerService::getOMX() {
    ALOGI("MediaPlayerService::getOMX");
 Mutex::Autolock autoLock(mLock);

 if (mOMX.get() == NULL) {
        mOMX = new OMX;
 }

 return mOMX;
}
