GenericSource: reset mDrmManagerClient when mDataSource is cleared.

Bug: 25070434
Change-Id: Iade3472c496ac42456e42db35e402f7b66416f5b
(cherry picked from commit b41fd0d4929f0a89811bafcc4fd944b128f00ce2)

void NuPlayer::GenericSource::prepareAsync() {
 if (mLooper == NULL) {
        mLooper = new ALooper;
        mLooper->setName("generic");
        mLooper->start();

        mLooper->registerHandler(this);
 }

    sp<AMessage> msg = new AMessage(kWhatPrepareAsync, id());
    msg->post();
}

void NuPlayer::GenericSource::prepareAsync() {
 if (mLooper == NULL) {
        mLooper = new ALooper;
        mLooper->setName("generic");
        mLooper->start();

        mLooper->registerHandler(this);
 }

    sp<AMessage> msg = new AMessage(kWhatPrepareAsync, id());
    msg->post();
}
