Add NPE check for refreshUi

The reason for NPE is that callback is registered too early, so
one callback is invoked before controller is fully inited. This
cl fix it in two ways:
1. Add NPE check in refreshUi, since this method in InstalledAppDetail
is robost to NPE
2. Move the callback to the end of constructor.

Bug: 37913054
Test: RunSettingsRoboTests
Change-Id: I54b03fb51a958a8e63bfe8a7ccfda79be1fa1956

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

@@ -344,6 +344,14 @@ public class AppButtonsPreferenceControllerTest {
         assertThat(controllable).isTrue();
     }
 
+    @Test
+    public void testRefreshUi_packageNull_shouldNotCrash() {
+        mController.mPackageName = null;
+
+        // Should not crash in this method
+        assertThat(mController.refreshUi()).isFalse();
+    }
+
     /**
      * The test fragment which implements
      * {@link ButtonActionDialogFragment.AppButtonsDialogListener}