Setup fbjni sub-project for maven (#119)

Summary:
Set up our fbjni sub-project to be published to Maven Central.
This removes a bunch of abiFilters that we no longer make use of, too.
Closes https://github.com/facebook/Sonar/pull/119

Reviewed By: priteshrnandgaonkar

Differential Revision: D8694537

Pulled By: passy

fbshipit-source-id: de246fbda99c02856fbc7806b78df2114cb82acb

android/third-party/RSocket/build.gradle

@@ -7,10 +7,6 @@ android {
     defaultConfig {
         minSdkVersion rootProject.minSdkVersion
         targetSdkVersion rootProject.targetSdkVersion
-        ndk {
-            abiFilters 'arm64-v8a', 'x86', 'armeabi-v7a'
-        }
-
         externalNativeBuild {
             cmake {
                 arguments '-DANDROID_TOOLCHAIN=clang'