Implement default assist app setting

- Add "None" support to AppListPreference
- Add DefaultAssistPreference to manage assist;
- Add AppListPreferenceWithSettings to show a settings icon;
- Implement DefaultAssistPreference based on AppListPreferenceWithSettings;
- Move voice input settings into ManageAssist and implement it
  based on AppListPreferenceWithSettings;

Bug:20210110
Change-Id: If283b8b55a46b428ecfa6e45dc2123292b1d4302
diff --git a/src/com/android/settings/Settings.java b/src/com/android/settings/Settings.java
index 5a6a2f0..5fb94f0 100644
--- a/src/com/android/settings/Settings.java
+++ b/src/com/android/settings/Settings.java
@@ -41,7 +41,6 @@
     public static class InputMethodAndLanguageSettingsActivity extends SettingsActivity { /* empty */ }
     public static class KeyboardLayoutPickerActivity extends SettingsActivity { /* empty */ }
     public static class InputMethodAndSubtypeEnablerActivity extends SettingsActivity { /* empty */ }
-    public static class VoiceInputSettingsActivity extends SettingsActivity { /* empty */ }
     public static class SpellCheckersSettingsActivity extends SettingsActivity { /* empty */ }
     public static class LocalePickerActivity extends SettingsActivity { /* empty */ }
     public static class UserDictionarySettingsActivity extends SettingsActivity { /* empty */ }
@@ -50,6 +49,7 @@
     public static class DeviceInfoSettingsActivity extends SettingsActivity { /* empty */ }
     public static class ApplicationSettingsActivity extends SettingsActivity { /* empty */ }
     public static class ManageApplicationsActivity extends SettingsActivity { /* empty */ }
+    public static class ManageAssistActivity extends SettingsActivity { /* empty */ }
     public static class AllApplicationsActivity extends SettingsActivity { /* empty */ }
     public static class HighPowerApplicationsActivity extends SettingsActivity { /* empty */ }
     public static class AppOpsSummaryActivity extends SettingsActivity {