DisLruCache 源码，摘录下来，方便查看。
https://android.googlesource.com/platform/libcore/+/android-4.1.1_r1/luni/src/main/java/libcore/io/DiskLruCache.java
https://github.com/JakeWharton/DiskLruCache/blob/master/src/main/java/com/jakewharton/disklrucache/DiskLruC
