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.xml b/java/res/layout/candidate.xml
index 5472a1d..aea34ac 100644
--- a/java/res/layout/candidate.xml
+++ b/java/res/layout/candidate.xml
@@ -42,10 +42,10 @@
         android:minWidth="@dimen/candidate_min_width"
         android:textSize="@dimen/candidate_text_size"
         android:textColor="@color/candidate_normal"
-        android:background="@drawable/btn_candidate"
         android:focusable="true"
         android:clickable="true"
-        android:gravity="center_vertical|center_horizontal" />
+        android:gravity="center_vertical|center_horizontal"
+        style="?attr/suggestionBackgroundStyle" />
     <TextView
         android:id="@+id/candidate_debug_info"
         android:layout_width="wrap_content"