Add plugin for LeakCanary 2 (#1959)

Summary:
Add plugin for LeakCanary 2 as requested various times: https://github.com/facebook/flipper/issues/1379 https://github.com/facebook/flipper/issues/832  https://github.com/square/leakcanary/issues/1777

## Changelog

* Adds a leakcanary2 plugin for Android
* Adds support for leakcanary2 to existing desktop plugin

Pull Request resolved: https://github.com/facebook/flipper/pull/1959

Test Plan:
* Docs updated to show new implementation
* Should old leakcanary plugin in sample be replaced?

Reviewed By: mweststrate

Differential Revision: D26691637

Pulled By: passy

fbshipit-source-id: 5e236fa6cc124f0720a6b21b5ee7c117ccf96fbf

android/plugins/leakcanary2/build.gradle

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+apply plugin: 'com.android.library'
+apply plugin: 'kotlin-android'
+apply plugin: 'kotlin-android-extensions'
+apply plugin: 'kotlin-kapt'
+apply plugin: 'maven'
+
+android {
+    compileSdkVersion rootProject.compileSdkVersion
+    buildToolsVersion rootProject.buildToolsVersion
+
+    defaultConfig {
+        minSdkVersion rootProject.minSdkVersion
+        targetSdkVersion rootProject.targetSdkVersion
+    }
+
+    dependencies {
+        implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$KOTLIN_VERSION"
+        implementation project(':android')
+        implementation deps.leakcanary2
+        compileOnly deps.jsr305
+    }
+}
+
+apply plugin: 'com.vanniktech.maven.publish'