use FgThread.class instead of UiThread.class for lock object

As is seen, UiThread/IoThread/DisplayThread/BackgroundThread
all use itself class for lock object. And for FgThread,
FgThread.class should be used for lock object

Test: null

Change-Id: I56e46fab33ff78d26b3e5197500764c763dba042
Signed-off-by: yangbingqian <yangbingqian@xiaomi.com>

services/core/java/com/android/server/FgThread.java

@@ -45,14 +45,14 @@ public final class FgThread extends ServiceThread {
     }
 
     public static FgThread get() {
-        synchronized (UiThread.class) {
+        synchronized (FgThread.class) {
             ensureThreadLocked();
             return sInstance;
         }
     }
 
     public static Handler getHandler() {
-        synchronized (UiThread.class) {
+        synchronized (FgThread.class) {
             ensureThreadLocked();
             return sHandler;
         }