fix gradle compliation (#925)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/925

Gradle is failing to compile yoga for two reasons:
1. Ever since `YogaNodeJNIPhantomRefs` was introduced which uses `DestructorThread.Destructor` from fbjni which was the first direct Java dependency from yoga java to fbjni java code.

2. Adding a missing gradle endpoint for `testutil` since it is now required for yoga unit-tests

Reviewed By: SidharthGuglani

Differential Revision: D17274226

fbshipit-source-id: 3df9648321162d34f81fd3675ca1474e8a1c6d3a

testutil/build.gradle

@@ -0,0 +1,38 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+apply plugin: 'com.android.library'
+
+android {
+    compileSdkVersion rootProject.compileSdkVersion
+    buildToolsVersion rootProject.buildToolsVersion
+
+    defaultConfig {
+        minSdkVersion rootProject.minSdkVersion
+        targetSdkVersion rootProject.targetSdkVersion
+
+        ndk {
+            abiFilters 'x86', 'x86_64', 'armeabi-v7a', 'arm64-v8a'
+        }
+
+        externalNativeBuild {
+            cmake {
+                arguments '-DANDROID_TOOLCHAIN=clang', '-DANDROID_STL=c++_static'
+            }
+        }
+    }
+
+    externalNativeBuild {
+        cmake {
+            path 'src/main/cpp/CMakeLists.txt'
+        }
+    }
+
+    dependencies {
+        implementation 'com.facebook.soloader:soloader:0.5.1'
+    }
+}