Add info string to advanced battery usage page

This CL adds an FYI string under the battery graph to let users
know that their current estimate is coming from the enhanced
estimate provider when it is enabled.

Test: Robotests
Bug: 38399654
Change-Id: If5cd622ef0251a5a483cef870fc2261369e14845

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

@@ -132,4 +132,9 @@ public class PowerUsageFeatureProviderImpl implements PowerUsageFeatureProvider
     public String getOldEstimateDebugString(String timeRemaining) {
         return null;
     }
+
+    @Override
+    public String getAdvancedUsageScreenInfoString() {
+        return null;
+    }
 }