Add support for null tag names from apps.

For Wakelocks and syncs there is no requirement that an app actually
supply a tag/name.  Jobs will always have system-supplied tags.

Bug: 26276001
Change-Id: Ie3b8e48a395de1506a2f15d5b9c9eab726bc6d0a

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

@@ -1745,6 +1745,9 @@ public class BatteryStatsImpl extends BatteryStats {
         }
 
         public void add(String name, T obj) {
+            if (name == null) {
+                name = "";
+            }
             mMap.put(name, obj);
             if (OVERFLOW_NAME.equals(name)) {
                 mCurOverflow = obj;
@@ -1776,6 +1779,9 @@ public class BatteryStatsImpl extends BatteryStats {
         }
 
         public T startObject(String name) {
+            if (name == null) {
+                name = "";
+            }
             T obj = mMap.get(name);
             if (obj != null) {
                 return obj;
@@ -1825,6 +1831,9 @@ public class BatteryStatsImpl extends BatteryStats {
         }
 
         public T stopObject(String name) {
+            if (name == null) {
+                name = "";
+            }
             T obj = mMap.get(name);
             if (obj != null) {
                 return obj;