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

build.gradle

@@ -15,6 +15,7 @@ buildscript {
         classpath 'com.android.tools.build:gradle:4.1.2'
         classpath 'com.vanniktech:gradle-maven-publish-plugin:0.14.2'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$KOTLIN_VERSION"
+        classpath "org.jetbrains.dokka:dokka-gradle-plugin:1.4.20"
     }
 }
 
@@ -101,6 +102,7 @@ ext.deps = [
         mockito            : 'org.mockito:mockito-core:2.26.0',
         okhttp3            : 'com.squareup.okhttp3:okhttp:3.14.1',
         leakcanary         : 'com.squareup.leakcanary:leakcanary-android:1.6.3',
+        leakcanary2        : 'com.squareup.leakcanary:leakcanary-android:2.6',
         testCore           : 'androidx.test:core:1.1.0',
         testRules          : 'androidx.test:rules:1.1.0',
         // Plugin dependencies