feat: add scrollbar style to candidate view

Currently candidate view use the default style in Android OS,
which varies in different manufacturer.  Adding a style to it
so that it has the same scrollbar on different Android model.

app/src/main/res/drawable/candidate_scrollbar_thumb.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle">
+    <size android:height="4dp" android:width="12dp" />
+    <solid android:color="#66666666" />
+    <corners android:radius="4dp"/>
+</shape>

app/src/main/res/layout/candidate_bar.xml

@@ -24,7 +24,7 @@
     android:id="@+id/candidate_root"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
-    android:orientation="vertical"
+    style="@style/candidate_scrollBar"
     android:fillViewport="true">
 
     <LinearLayout

app/src/main/res/values/styles.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <style name="candidate_scrollBar">
+        <item name="android:scrollbarSize">4dp</item>
+        <item name="android:scrollbarStyle">outsideOverlay</item>
+        <item name="android:scrollbars">horizontal</item>
+        <item name="android:fadeScrollbars">true</item>
+        <item name="android:scrollbarThumbHorizontal">@drawable/candidate_scrollbar_thumb</item>
+    </style>
+</resources>