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/PowerUsageFeatureProvider.java

@@ -113,4 +113,10 @@ public interface PowerUsageFeatureProvider {
      * @return A string containing the estimate and a label indicating it is a normal estimate
      */
     String getOldEstimateDebugString(String timeRemaining);
+
+    /**
+     * Returns the string to show in the advanced usage battery page when enhanced estimates are
+     * enabled. This string notifies users that the estimate is using enhanced prediction.
+     */
+    String getAdvancedUsageScreenInfoString();
 }