Messaging: Style the attachment choosers properly

On light background they stand out lighter, but on dark they just blend
in - we can reuse the attachment background to fix that!

Change-Id: I0e0f09d0cd305ce1bc8aa07c3a20b79993d40e60

res/layout/mediapicker_audio_chooser.xml

@@ -19,7 +19,7 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:background="@android:color/white"
+    android:background="@color/attachment_bg_color"
     android:importantForAccessibility="no" >
 
     <FrameLayout

res/layout/mediapicker_camera_chooser.xml

@@ -20,7 +20,7 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:background="@android:color/white">
+    android:background="@color/attachment_bg_color">
 
     <androidx.constraintlayout.widget.ConstraintLayout
         android:id="@+id/mediapicker_enabled"

res/layout/mediapicker_contact_chooser.xml

@@ -20,7 +20,7 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:background="@android:color/white"
+    android:background="@color/attachment_bg_color"
     android:importantForAccessibility="no" >
 
     <FrameLayout

res/layout/mediapicker_gallery_chooser.xml

@@ -20,7 +20,7 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:background="@android:color/white">
+    android:background="@color/attachment_bg_color">
 
     <FrameLayout
         android:id="@+id/mediapicker_enabled"