Add metric for new battery relative functions for S (1/2)

Reference the table from PM requested:
https://docs.google.com/spreadsheets/d/1TC_6MLVMvOS0p1ic0wPfQ-19AOuY8Kc8FUJdqaQSM4I/edit?ts=6098b296#gid=0

Bug: 188505537
Test: make SettingsRoboTests
Change-Id: I2a4df9ab900cbeaf9885bb2dbc21ea848920bc81

src/com/android/settings/fuelgauge/BatteryOptimizeUtils.java

@@ -27,6 +27,7 @@ import com.android.settingslib.fuelgauge.PowerAllowlistBackend;
 /** A utility class for application usage operation. */
 public class BatteryOptimizeUtils {
     private static final String TAG = "BatteryOptimizeUtils";
+    private static final String UNKNOWN_PACKAGE = "unknown";
 
     @VisibleForTesting AppOpsManager mAppOpsManager;
     @VisibleForTesting BatteryUtils mBatteryUtils;
@@ -108,6 +109,10 @@ public class BatteryOptimizeUtils {
                 || mPowerAllowListBackend.isDefaultActiveApp(mPackageName);
     }
 
+    String getPackageName() {
+        return mPackageName == null ? UNKNOWN_PACKAGE : mPackageName;
+    }
+
     private void refreshState() {
         mPowerAllowListBackend.refreshList();
         mAllowListed = mPowerAllowListBackend.isAllowlisted(mPackageName);