Switch Process.myTid over to the libcore JNI too.

Change-Id: I80eaecff014fabf809f4be3c9ee8b26aa64bf0cf
diff --git a/core/java/android/os/Process.java b/core/java/android/os/Process.java
index 0e38ebc..ebeb9f8 100644
--- a/core/java/android/os/Process.java
+++ b/core/java/android/os/Process.java
@@ -657,7 +657,9 @@
      * Returns the identifier of the calling thread, which be used with
      * {@link #setThreadPriority(int, int)}.
      */
-    public static final native int myTid();
+    public static final int myTid() {
+        return Libcore.os.gettid();
+    }
 
     /**
      * Returns the identifier of this process's uid.  This is the kernel uid