update gradle plugin

android/build.gradle

@@ -5,7 +5,7 @@ buildscript {
         google()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.2.1'
+        classpath 'com.android.tools.build:gradle:3.6.0'
     }
 }
 

docs/building.md

@@ -260,7 +260,7 @@ In order to build the Android APK file, you need:
 
 1.  the [Android Studio](https://developer.android.com/studio/) with API 26 support;
 2.  the [Android NDK](http://developer.android.com/tools/sdk/ndk/index.html);
-3.  Gradle 3.2.1 or later.
+3.  Gradle 5.6.4 or later.
 
 ### Building with Gradle