Add isSliceable API to BasePrefController

Only support explicitly approved Settings Slices,
dictated by controllers which return true for the new
method isSliceable.

Updating the supported settings to a whitelist means that
the method to return all available slices must be updated,
and checking slicability when we index slices.

Test: robotests
Change-Id: I85848c2cdf3e151fa94b33dd1dc5c0374ef94b5b
Merged-In: Ib2b9690cdd0036b5cc4a1cb846c52bce7c824ab9
Fixes: 79779103

src/com/android/settings/notification/MediaVolumePreferenceController.java

@@ -18,6 +18,7 @@ package com.android.settings.notification;
 
 import android.content.Context;
 import android.media.AudioManager;
+import android.text.TextUtils;
 
 import com.android.settings.R;
 
@@ -37,6 +38,11 @@ public class MediaVolumePreferenceController extends
                 : UNSUPPORTED_ON_DEVICE;
     }
 
+    @Override
+    public boolean isSliceable() {
+        return TextUtils.equals(getPreferenceKey(), KEY_MEDIA_VOLUME);
+    }
+
     @Override
     public String getPreferenceKey() {
         return KEY_MEDIA_VOLUME;