Bump Gradle plugin to 3.6.1 (#958)

Summary:
This is in line with the most recent stable Android Studio Release.
Pull Request resolved: https://github.com/facebook/flipper/pull/958

Test Plan:
Used it myself.

Open Source CI required a higher NDK, so let's first check what CI says to that internally now.

Reviewed By: jknoxville

Differential Revision: D20794634

Pulled By: passy

fbshipit-source-id: c32f934634b036ad3c1cad9fc49541e585d64329

build.gradle

@@ -11,7 +11,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.5.2'
+        classpath 'com.android.tools.build:gradle:3.6.1'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$KOTLIN_VERSION"
     }
 }
@@ -44,6 +44,7 @@ ext {
     targetSdkVersion = 29
     compileSdkVersion = 29
     buildToolsVersion = '29.0.2'
+    ndkVersion = '20.0.5594570'
     sourceCompatibilityVersion = JavaVersion.VERSION_1_7
     targetCompatibilityVersion = JavaVersion.VERSION_1_7
 }