Generic Mutable class for HIDL code.

In general, using Mutable inside of structures
or as API arguments is a code smell. However, it
is the most syntactically clean way of fetching
something out of a lambda or inner class.

Using this, one can do:
Mutable<Result> res = new Mutable<>();
hidlObject.foo((...) -> {
    res.value = ...;
});

There is an alternative:
Result a[] = new Result[1];
hidlObject.foo((...) -> {
    a[0] = ...;
});

However, this alternative syntax is relatively messy.

Bug: N/A
Test: boot, use w/ wifi
Change-Id: Ibff13c653cc17bd25ddbb0534ba21ef485bff7aa

core/java/android/os/HidlSupport.java

@@ -85,6 +85,25 @@ public class HidlSupport {
         return lft.equals(rgt);
     }
 
+    /**
+     * Class which can be used to fetch an object out of a lambda. Fetching an object
+     * out of a local scope with HIDL is a common operation (although usually it can
+     * and should be avoided).
+     *
+     * @param <E> Inner object type.
+     */
+    public static final class Mutable<E> {
+        public E value;
+
+        public Mutable() {
+            value = null;
+        }
+
+        public Mutable(E value) {
+            this.value = value;
+        }
+    }
+
     /**
      * Similar to Arrays.deepHashCode, but also take care of lists.
      */