Fix LeakCanary plugin UI, Upgrade LC dependency and make it compile only (#3367)

Summary:
**Currently, the Leak Canary plugins is broken, see this screenshot (unable to expand):**
<img width="1359" alt="Screen Shot 2022-01-29 at 2 06 33 PM" src="https://user-images.githubusercontent.com/745166/151679662-9d8e8eb6-c19c-4008-9821-1c2c1af92351.png">

**This fix repairs and improves the UI:**
<img width="1350" alt="Screen Shot 2022-01-29 at 2 06 03 PM" src="https://user-images.githubusercontent.com/745166/151679677-7965604c-dd04-4ee5-bc8e-d53a82da1bd7.png">

## Changelog

- Fixes UI to enable showing leak trace
- Upgrades LackCanary dependency from 2.6 to 2.8.1
- Makes Kotlin and LC `compileOnly` dependencies in order to prevent conflict for the users
- Updates documentation

Pull Request resolved: https://github.com/facebook/flipper/pull/3367

Test Plan:
See demo at https://github.com/hbmartin/leakcanary/tree/flipper-demo/leakcanary-android-sample

**Static Docs Preview: flipper**
|[Full Site](https://our.intern.facebook.com/intern/staticdocs/eph/D33915179/V3/flipper/)|

|**Modified Pages**|

Reviewed By: passy

Differential Revision: D33915179

Pulled By: cekkaewnumchai

fbshipit-source-id: 9698dba23ab475c8cd84e4c222dfc41712b05a1e

build.gradle

@@ -96,7 +96,7 @@ ext.deps = [
         mockito            : 'org.mockito:mockito-core:4.3.1',
         okhttp3            : 'com.squareup.okhttp3:okhttp:4.9.3',
         leakcanary         : 'com.squareup.leakcanary:leakcanary-android:1.6.3',
-        leakcanary2        : 'com.squareup.leakcanary:leakcanary-android:2.6',
+        leakcanary2        : 'com.squareup.leakcanary:leakcanary-android:2.8.1',
         protobuf           : 'com.google.protobuf:protobuf-java:3.19.4',
         testCore           : 'androidx.test:core:1.4.0',
         testRules          : 'androidx.test:rules:1.4.0',