Use the background thread instead ActivityManager to update CPU stats.

Updating the CPU stats can be quite slow and blocks the ActivityManager
handler thread.

Bug: 115506775
Change-Id: I9f67acfe2724c11d117703b2c4ccdbcc1f61c5ab
Test: manual - dumpsys batterystats | grep CPU

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

@@ -337,6 +337,9 @@ public class BatteryStatsImpl extends BatteryStats {
 
     private final PlatformIdleStateCallback mPlatformIdleStateCallback;
 
+    /**
+     * This handler is running on {@link BackgroundThread}.
+     */
     final class MyHandler extends Handler {
         public MyHandler(Looper looper) {
             super(looper, null, true);