Fix the GAR issue of TalkBack said "double-tap to activate" on the

slider title of Accessibility button page.

Root cause:
Set "android:selectable" as true.

Solution:
Remove "android:selectable".

Bug: 183469419
Test: manual test
Change-Id: I337cfe0795205dec5e4654b90d6c7e2c7fd7ca77

res/xml/accessibility_button_settings.xml

@@ -56,7 +56,6 @@
     <com.android.settings.widget.SeekBarPreference
         android:key="accessibility_button_opacity"
         android:title="@string/accessibility_button_opacity_title"
-        android:selectable="true"
         android:persistent="false"
         settings:controller="com.android.settings.accessibility.FloatingMenuOpacityPreferenceController"/>