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

docs/setup/leak-canary-2-plugin.mdx

@@ -0,0 +1,49 @@
+---
+id: leak-canary-plugin
+title: LeakCanary Setup
+sidebar_label: LeakCanary
+---
+
+Ensure that you already have an explicit dependency in your application's
+`build.gradle` including the plugin dependency, e.g.
+
+```groovy
+dependencies {
+  debugImplementation 'com.facebook.flipper:flipper-leakcanary2-plugin:0.76.0'
+  debugImplementation 'com.squareup.leakcanary:leakcanary-android:2.6'
+}
+```
+
+Update your the `onCreate` method in you `Application` to add the LeakCanary2 plugin to Flipper and the Flipper listener to LeakCanary
+
+```kt
+import com.facebook.flipper.plugins.leakcanary2.FlipperLeakListener
+import com.facebook.flipper.plugins.leakcanary2.LeakCanary2FlipperPlugin
+
+...
+
+  override fun onCreate() {
+    super.onCreate()
+    setupFlipper()
+
+    /*
+    set the flipper listener in leak canary config
+    */
+    LeakCanary.config = LeakCanary.config.copy(
+        onHeapAnalyzedListener = FlipperLeakListener()
+    )
+
+    SoLoader.init(this, false)
+
+    if (BuildConfig.DEBUG && FlipperUtils.shouldEnableFlipper(this)) {
+      val client = AndroidFlipperClient.getInstance(this)
+      /*
+      add leak canary plugin to flipper
+      */
+      client.addPlugin(LeakCanary2FlipperPlugin())
+      client.start()
+    }
+  }
+```
+
+That's it!