Upgrade to OkHttp3

Summary:
Update to [OkHttp](https://github.com/square/okhttp) to [OkHttp3](https://publicobject.com/2015/12/12/com-squareup-okhttp3/)

We must also update:
- Fresco to 0.10.0
- okio to 1.8.0

**Motivation**
Reasons for upgrading:
* Issue #4021
* "We discovered that RN Android sometimes fails to connect to the latest stable version of NGINX when HTTP/2 is enabled. We aren't seeing errors with other HTTP clients so we think it's specific to RN and OkHttp. Square has fixed several HTTP/2 bugs over the past eight months." - ide
* OkHttp3 will be maintained & improved, but OkHttp2 will only receive [security fixes](https://publicobject.com/2016/02/11/okhttp-certificate-pinning-vulnerability/)
* Cleaner APIs - "Get and Set prefixes are avoided"
* Deprecated/Removed - HttpURLConnection & Apache HTTP
* React Native apps are currently being forced to bundle two versions of OkHttp (v2 & v3), if another library uses v3
* Improved WebSocket performance - [CHANGELOG.md](https://github.com/square/okhttp/blob/master
Closes https://github.com/facebook/react-native/pull/6113

Reviewed By: andreicoman11, lexs

Differential Revision: D3292375

Pulled By: bestander

fbshipit-source-id: 7c7043eaa2ea63f95854108b401c4066098d67f7

local-cli/generator-android/templates/src/app/proguard-rules.pro

@@ -51,9 +51,9 @@
 
 -keepattributes Signature
 -keepattributes *Annotation*
--keep class com.squareup.okhttp.** { *; }
--keep interface com.squareup.okhttp.** { *; }
--dontwarn com.squareup.okhttp.**
+-keep class okhttp3.** { *; }
+-keep interface okhttp3.** { *; }
+-dontwarn okhttp3.**
 
 # okio