Upgrade playground app's Gradle to 5.1.1 (matches version of lib) (#5491)

playground/android/build.gradle

@@ -11,7 +11,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.3.2'
+        classpath 'com.android.tools.build:gradle:3.4.1'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
 
         // NOTE: Do not place your application dependencies here; they belong

playground/android/gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Sun Aug 11 15:52:47 IDT 2019
+#Mon Sep 16 17:06:02 IDT 2019
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.10.2-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.1.1-all.zip