Summary:
This is an incomplete effort to migrate from libfb to libfbjni. This is needed to restore the compatibility with Flipper and other FB Android projects that make use of FBJNI. Effectively, the outcome is that `fbjni` no longer has a checked-in copy here, but instead relies on the public artifacts published at github.com/facebookincubator/fbjni that can be deduplicated at build-time.

**A non-exhaustive list of tasks:**

* [X] Gradle builds the SDK and RNTester for Android.
* [X] Buck build for rntester works in OSS.
* [ ] Move from `java-only` release to full `fbjni` release. This requires finding a solution for stripping out `.so` files that the old `Android.mk` insists on including in the final artifacts and will clash with the full distribution.
* [ ] Import this and fix potential internal build issues.
* [ ] Verify that the changes made to the Hermes integration don't have any unintended consequences.

## Changelog

[Android] [Changed] - Migrated from libfb to libfbjni for JNI calls
Pull Request resolved: https://github.com/facebook/react-native/pull/27729

Test Plan:
- CI is already passing again for Gradle and Buck in OSS.
- After applying the following patch, RNTester builds and works with the latest Flipper SDK:

```
 diff --git a/RNTester/android/app/build.gradle b/RNTester/android/app/build.gradle
index b8a6437d7..eac942104 100644
 --- a/RNTester/android/app/build.gradle
+++ b/RNTester/android/app/build.gradle
@@ -170,10 +170,19 @@ dependencies {
     debugImplementation files(hermesPath + "hermes-debug.aar")
     releaseImplementation files(hermesPath + "hermes-release.aar")

-    debugImplementation("com.facebook.flipper:flipper:0.23.4") {
+    debugImplementation("com.facebook.flipper:flipper:+") {
         exclude group:'com.facebook.yoga'
-        exclude group:'com.facebook.flipper', module: 'fbjni'
-        exclude group:'com.facebook.litho', module: 'litho-annotations'
+        exclude group:'com.facebook.fbjni'
+    }
+
+    debugImplementation("com.facebook.flipper:flipper-network-plugin:+") {
+        exclude group:'com.facebook.yoga'
+        exclude group:'com.facebook.fbjni'
+    }
+
+    debugImplementation("com.facebook.flipper:flipper-fresco-plugin:+") {
+        exclude group:'com.facebook.yoga'
+        exclude group:'com.facebook.fbjni'
     }

     if (useIntlJsc) {
```

Reviewed By: mdvacca

Differential Revision: D19345270

Pulled By: passy

fbshipit-source-id: 33811e7f97f44f2ec5999e1c35339909dc4fd3b1