private int getFreeMemoryCacheBytes() {
        return memoryCache.getMaxSize() - memoryCache.getCurrentSize();
    }
Attempts to allocate {@link Bitmap}s and returns {@code true} if there are more
{@link Bitmap}s to allocate and {@code false} otherwise.
core/src/main/java/com/bumptech/glide/load/engine/prefill/BitmapPreFillRunner.java