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/res/xml/manage_assist.xml b/res/xml/manage_assist.xml
index e3ee859..39a7120 100644
--- a/res/xml/manage_assist.xml
+++ b/res/xml/manage_assist.xml
@@ -19,12 +19,23 @@
         xmlns:android="http://schemas.android.com/apk/res/android"
         android:key="manage_assist">
 
+    <com.android.settings.applications.DefaultAssistPreference
+            android:key="default_assist"
+            android:title="@string/default_assist_title"
+            android:summary="@string/default_assist_none"
+            />
+
     <SwitchPreference
         android:key="context"
         android:title="@string/assist_access_context_title"
         android:summary="@string/assist_access_context_summary"
         android:persistent="false"/>
 
+    <com.android.settings.voice.VoiceInputListPreference
+            android:key="voice_input_settings"
+            android:title="@string/voice_input_settings_title"
+            />
+
     <Preference
         android:summary="@string/assist_footer"
         android:selectable="false"/>