Plumb context object to PowerWhitelistBackend.

Change-Id: Ice0d037625d47f1f668b00499a7f1ad2302cc0ba
Fixes: 76435804
Test: robotests

tests/robotests/src/com/android/settings/fuelgauge/BatteryOptimizationPreferenceControllerTest.java

@@ -115,6 +115,10 @@ public class BatteryOptimizationPreferenceControllerTest {
      */
     public static class TestPowerWhitelistBackend extends PowerWhitelistBackend {
 
+        public TestPowerWhitelistBackend(Context context) {
+            super(context);
+        }
+
         @Override
         public void refreshList() {
             // Do nothing so we could mock it without error