Refactor bottom spacer fill colour to constant

Author: yawnoc

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

@@ -70,6 +70,6 @@
     android:id="@+id/bottom_spacer"
     android:layout_width="match_parent"
     android:layout_height="0dp"
-    android:background="@android:color/black"
+    android:background="@color/bottom_spacer_fill"
   />
 </io.github.yawnoc.strokeinput.InputContainer>

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

@@ -21,4 +21,6 @@
   <color name="candidate_button_text_pressed">@android:color/black</color>
   <color name="candidate_button_fill_unpressed">@android:color/black</color>
   <color name="candidate_button_fill_pressed">@color/umbrella_orange</color>
+  <!-- Bottom spacer -->
+  <color name="bottom_spacer_fill">@android:color/black</color>
 </resources>