Add javadoc saying that wake locks are ref counted by default.

Bug: 3012561
Change-Id: I441a42fdb12ba6d87d6f5a49463d6ddcc300acb7
diff --git a/core/java/android/os/PowerManager.java b/core/java/android/os/PowerManager.java
index 3876a3e..156da47 100644
--- a/core/java/android/os/PowerManager.java
+++ b/core/java/android/os/PowerManager.java
@@ -232,6 +232,8 @@
         /**
          * Sets whether this WakeLock is ref counted.
          *
+         * <p>Wake locks are reference counted by default.
+         *
          * @param value true for ref counted, false for not ref counted.
          */
         public void setReferenceCounted(boolean value)