updated gradle build tool version to 3.1.0 and gradle to 4.6

Summary: Upgraded gradle build tool version to 3.1.0 to solve issue "No toolchains found in the NDK toolchains folder for ABI with prefix: mips-linux-android"

Reviewed By: davidaurelio

Differential Revision: D14042022

fbshipit-source-id: c615127fa296f30f589b183bffc8f2a95f7b943b

build.gradle

@@ -9,11 +9,12 @@
 
 buildscript {
     repositories {
+        google()
         jcenter()
         maven { url 'https://maven.google.com/' }
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.0.0'
+        classpath 'com.android.tools.build:gradle:3.1.0'
         classpath 'com.jfrog.bintray.gradle:gradle-bintray-plugin:1.7.3'
         classpath 'com.github.dcendents:android-maven-gradle-plugin:2.0'
 
@@ -24,6 +25,7 @@ buildscript {
 
 allprojects {
   repositories {
+    google()
     jcenter()
   }
 }