Add ability to guard a thread against setting its own prio to bg

The guard is compiled out by default because it adds overhead to
android.os.Process.setPriority().

Change-Id: Ibb2a648c6349b381abb7ae62a358888b04fba871
diff --git a/core/java/android/os/Process.java b/core/java/android/os/Process.java
index 5640a06..f695dbb 100644
--- a/core/java/android/os/Process.java
+++ b/core/java/android/os/Process.java
@@ -626,6 +626,15 @@
             throws IllegalArgumentException, SecurityException;
 
     /**
+     * Call with 'false' to cause future calls to {@link #setThreadPriority(int)} to
+     * throw an exception if passed a background-level thread priority.  This is only
+     * effective if the JNI layer is built with GUARD_THREAD_PRIORITY defined to 1.
+     *
+     * @hide
+     */
+    public static final native void setCanSelfBackground(boolean backgroundOk);
+
+    /**
      * Sets the scheduling group for a thread.
      * @hide
      * @param tid The indentifier of the thread/process to change.