Fix talkback speaks "double tap to activate" on preference

Talkback will speak "double tap to activate" on clickable view.
But the SliderPreference is not clickable. Because it is default
clickable for Preference. We should set android:selectable in every
SliderPreference.

Change-Id: I7bf09c2a7e2d8688352e35c1882674b6e73e5db7
Fixes: 121111172
Test: Manual

src/com/android/settings/widget/SeekBarPreference.java

@@ -91,6 +91,11 @@ public class SeekBarPreference extends RestrictedPreference
         notifyChanged();
     }
 
+    @Override
+    public boolean isSelectable() {
+        return false;
+    }
+
     @Override
     public void onBindViewHolder(PreferenceViewHolder view) {
         super.onBindViewHolder(view);