Bump Gradle plugin version in Playground (#6138)

Set ndkVersion explicitly in build.gradle because https://stackoverflow.com/questions/60404457/android-no-version-of-ndk-matched-the-requested-version

playground/android/app/build.gradle

 
 android {
     compileSdkVersion 28
+    ndkVersion "20.1.5948944"
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8

playground/android/build.gradle

     }
     dependencies {
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
-        classpath 'com.android.tools.build:gradle:3.5.3'
+        classpath 'com.android.tools.build:gradle:3.6.3'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files

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

-#Mon Sep 16 17:06:02 IDT 2019
+#Sun Apr 19 12:17:05 IDT 2020
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.5-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.6.4-all.zip