MediaExtractor: clarify EOS behavior for local files

    /**

     * Advance to the next sample. Returns false if no more sample data

     * is available (end of stream).

     *

     * When extracting a local file, the behaviors of {@link #advance} and

     * {@link #readSampleData} are undefined in presence of concurrent

     * writes to the same local file; more specifically, end of stream

     * could be signalled earlier than expected.

     */

    public native boolean advance();