Fix 5797764: Increase lock screen's thread priority

This increases lock screen's thread priority from THREAD_PRIORITY_FOREGROUND
to THREAD_PRIORITY_DISPLAY to ensure it runs before other activities that
might stall lock screen when the screen turns on.

Change-Id: I14cf9f3f5c092817bc6cf2d0a254001a5d34f744
diff --git a/services/java/com/android/server/wm/WindowManagerService.java b/services/java/com/android/server/wm/WindowManagerService.java
index 4dcf59b..e8c0516 100644
--- a/services/java/com/android/server/wm/WindowManagerService.java
+++ b/services/java/com/android/server/wm/WindowManagerService.java
@@ -728,7 +728,7 @@
             //Looper.myLooper().setMessageLogging(new LogPrinter(
             //        Log.VERBOSE, "WindowManagerPolicy", Log.LOG_ID_SYSTEM));
             android.os.Process.setThreadPriority(
-                    android.os.Process.THREAD_PRIORITY_FOREGROUND);
+                    android.os.Process.THREAD_PRIORITY_DISPLAY);
             android.os.Process.setCanSelfBackground(false);
             mPolicy.init(mContext, mService, mService, mPM);