thumbnail的處理流程和讀取縮圖thumbnail所在sample的index的方法

取得video thumbnail的處理流程如所示，取得image的thumbnail的處理流程類似，只是取得image thumbnail的函數變成了ThumbnailUtils.createImageThumbnail。

Thumbnail的傳回型別是Bitmap。

尋找縮圖所在sample的index的方法在SampleTable::findThumbnailSample函數中，在要尋找的前numSamplesToScan個samples 中，尋找size最大的sample作為縮圖的sample index。

status_t SampleTable::findThumbnailSample(uint32_t *sample_index) {    Mutex::Autolock autoLock(mLock);    if (mSyncSampleOffset < 0) {        // All samples are sync-samples.        *sample_index = 0;        return OK;    }    uint32_t bestSampleIndex = 0;    size_t maxSampleSize = 0;    static const size_t kMaxNumSyncSamplesToScan = 20; // 最大尋找20個samples    // Consider the first kMaxNumSyncSamplesToScan sync samples and    // pick the one with the largest (compressed) size as the thumbnail.    size_t numSamplesToScan = mNumSyncSamples;    if (numSamplesToScan > kMaxNumSyncSamplesToScan) {        numSamplesToScan = kMaxNumSyncSamplesToScan;    }    for (size_t i = 0; i < numSamplesToScan; ++i) {        uint32_t x = mSyncSamples[i];        // Now x is a sample index.        size_t sampleSize;        status_t err = getSampleSize_l(x, &sampleSize);        if (err != OK) {            return err;        }        if (i == 0 || sampleSize > maxSampleSize) { // 尋找size最大的一個sample當做thumbnail的sample            bestSampleIndex = x;            maxSampleSize = sampleSize;        }    }    *sample_index = bestSampleIndex;    return OK;}