Upgrade gradle and gradle plugin (#1327)

Summary:
Android Studio is on 4.0 now, we need the plugin for proper support.
Pull Request resolved: https://github.com/facebook/flipper/pull/1327

Test Plan: Synced and build with it.

Reviewed By: nikoant

Differential Revision: D22356268

Pulled By: passy

fbshipit-source-id: 63c7b727e9a0eaaea7beed1f9819c1e2d15a0ff4

build.gradle

@@ -11,7 +11,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.6.1'
+        classpath 'com.android.tools.build:gradle:4.0.0'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$KOTLIN_VERSION"
     }
 }