Make the default getTimeSpentInApp() return empty

Current ApplicationFeatureProvider.getTimeSpentInApp() defaults returns
null, in extreme case there is ACTION_APP_USAGE_SETTINGS handler on
device but getTimeSpentInApp() still returns null, may lead
AppTimeSpentPreference to crash.

Make it returns empty string instead to eliminate this extreme case.

Bug: 236346018
Test: Unit test & Manual with SpaActivity
Change-Id: I33b413ceb9bdf48e3902e02c91fab617fe297862

src/com/android/settings/applications/ApplicationFeatureProvider.java

@@ -19,6 +19,8 @@ package com.android.settings.applications;
 import android.annotation.UserIdInt;
 import android.content.Intent;
 
+import androidx.annotation.NonNull;
+
 import java.util.List;
 import java.util.Set;
 
@@ -85,8 +87,9 @@ public interface ApplicationFeatureProvider {
      * Returns a user readable text explaining how much time user has spent in an app at a
      * pre-specified duration.
      */
+    @NonNull
     default CharSequence getTimeSpentInApp(String packageName) {
-        return null;
+        return "";
     }
 
     /**

tests/spa_unit/src/com/android/settings/spa/app/appinfo/AppTimeSpentPreferenceTest.kt

@@ -35,13 +35,11 @@ import androidx.test.core.app.ApplicationProvider
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import com.android.settings.R
 import com.android.settings.testutils.FakeFeatureFactory
-import com.android.settingslib.spaprivileged.framework.common.storageStatsManager
 import org.junit.Before
 import org.junit.Rule
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.mockito.Mock
-import org.mockito.Mockito
 import org.mockito.Mockito.any
 import org.mockito.Mockito.anyInt
 import org.mockito.Spy