narrow fbcore proguard rule (#2519)
narrow fbcore proguard rule (#2519)

Summary:
## Motivation (required)

Close https://github.com/facebook/fresco/issues/2499

## Test Plan (required)

1. Create `aar` and do `unzip -l fbcore.aar` to check if the rule is already changed
2. Build `normalRelease` flavour of `samples:showcase` and test some features
3. Build `samples:animation2` build type `debug` and minify enabled

For 2 and 3 I added `proguard-rules.pro` in `proguardFiles` just to make sure the proguard from aar is included

Pull Request resolved: https://github.com/facebook/fresco/pull/2519

Reviewed By: defHLT

Differential Revision: D23212514

Pulled By: oprisnik

fbshipit-source-id: dfa5c574b2d5bcdea62c3291b027be0b5419ff6a

facebook/fresco,facebook/fresco,facebook/fresco,facebook/fresco,facebook/fresco,facebook/fresco