protected int sizeOf(K key, V value) {
        return 1;
    }
Returns the size of the entry for {@code key} and {@code value} in
user-defined units.  The default implementation returns 1 so that size
is the number of entries and max size is the maximum number of entries.

<p>An entry's size must not change while it is in the cache.
VideoSqlLite/src/main/java/com/yc/videosqllite/cache/SystemLruCache.java