Tell Fresco to use disk cache for thumbnails of local videos

### Description

I'm using version 1.10.0

- In root application, I call `Fresco.initialize(getApplicationContext())`.
- In other UI controls, I pass `Uri.fromFile("file://.../sdcard/example-video.mp4")` to `SimpleDraweeView`. The view generates in-memory thumbnail and displays it correctly.
- I check `/data/data/my.app.id/cache/image_cache/` in emulator, no files or directories are in there. Which means for local videos, Fresco does not use disk cache.

### Additional Information

* Fresco version: 1.10.0
* Platform version: emulator API 23

May I ask if you could consider adding option to tell Fresco to use disk cache for local videos? Normally generating thumbnails takes time, like 1-2 seconds per video (on my real device). So disk cache would help a lot.

Thank you,

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P.S: If someone could tell me how to do this hack then it would also help a lot:

Would it be possible to force Fresco to flush the in-memory generated thumbnails to disk cache? Then next time it checks for cache, it can use the cached files.