build: update gradle

Change-Id: Ie9b2afb815355532d2c0df8b821100904da17288

ring-android/build.gradle

@@ -4,7 +4,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.3.0'
+        classpath 'com.android.tools.build:gradle:3.3.1'
         classpath 'com.google.gms:google-services:4.2.0'
     }
 }