Use custom theme for Keyboard theme switch

This change reduces the number of layout files and make these
theme-generic.  This might be very helpful to re-design suggestion
strip.

Bug: 4175031
Change-Id: Idca10a4aa0bf7ac496eedd1879311e59780a423b
diff --git a/java/res/layout/candidate_preview.xml b/java/res/layout/candidate_preview.xml
index fe2002d..32705c9 100644
--- a/java/res/layout/candidate_preview.xml
+++ b/java/res/layout/candidate_preview.xml
@@ -19,11 +19,10 @@
 -->
 
 <TextView xmlns:android="http://schemas.android.com/apk/res/android"
-    android:layout_width="wrap_content" 
+    android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:textSize="18sp"
     android:textColor="?android:attr/textColorPrimaryInverse"
     android:minWidth="32dip"
     android:gravity="center"
-    android:background="@drawable/candidate_feedback_background"
-    />
+    style="?attr/suggestionPreviewBackgroundStyle" />