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']
+        }
+    }
+}
+