Messaging: Set input text box background

Fixes the background in dark mode. Also modify the color for light
mode to be a very light shade of the neutral color.

Change-Id: I8a420405dcc478f5d50bf9b1d4739a58f8cd976a

res/drawable/msg_bubble_input_rounded.xml

@@ -8,6 +8,6 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:shape="rectangle">
 
-    <solid android:color="@android:color/white" />
+    <solid android:color="@color/compose_send_text_background_color" />
     <corners android:radius="28dp" />
 </shape>