Add Unsafe based storage backend

Summary:
@public

Adds another version of property storage for `YogaNode`, using `sun.misc.Unsafe`.

Adopts the stub concept from Litho for `Unsafe`, as it is hidden by the Android SDK.

Reviewed By: pasqualeanatriello

Differential Revision: D9140103

fbshipit-source-id: a4b376eca341b724a00f873467ae8bf8eaac69f4

java/stubs/build.gradle

@@ -0,0 +1,10 @@
+apply plugin: 'java'
+
+sourceSets {
+    main {
+        java {
+            srcDirs = ['src']
+        }
+    }
+}
+

java/stubs/src/sun/misc/Unsafe.java

@@ -0,0 +1,27 @@
+/*
+ *  Copyright (c) 2018-present, Facebook, Inc.
+ *
+ *  This source code is licensed under the MIT license found in the LICENSE
+ *  file in the root directory of this source tree.
+ *
+ */
+
+package sun.misc;
+
+/**
+ * Stub for sun.misc.Unsafe, which is not exposed by the Android SDK.
+ *
+ * This only contains the methods and fields we need for Yoga.
+ */
+public final class Unsafe {
+  private static final Unsafe theUnsafe = null;
+
+  public final int getInt(Object object, long offset) {
+    throw new RuntimeException("Stub!");
+  }
+
+  public final void putInt(Object object, long offset, int value) {
+    throw new RuntimeException("Stub!");
+  }
+}
+