Change shared looper tag from ACTIVITY_MANAGER to SYSTEM_SERVER

Most of these are shared, so enabling all ActivityManager logging just
to get looper traces seems wrong.

Test: build, trace with system_server tag, see looper info
Change-Id: I3741dbacab25f5c02f6f8d17f0c8377d8f842776

core/java/com/android/internal/os/BackgroundThread.java

@@ -35,7 +35,7 @@ public final class BackgroundThread extends HandlerThread {
         if (sInstance == null) {
             sInstance = new BackgroundThread();
             sInstance.start();
-            sInstance.getLooper().setTraceTag(Trace.TRACE_TAG_ACTIVITY_MANAGER);
+            sInstance.getLooper().setTraceTag(Trace.TRACE_TAG_SYSTEM_SERVER);
             sHandler = new Handler(sInstance.getLooper());
         }
     }