chromiumos-asset: change boot message background to menu ui style

Change boot message background to dark color, and rename old *.png
to *_light.png for backup.

BUG=b:149889061
TEST=manually copy the files to DUT and run
     - `display_boot_message <message> en`
     - `display_boot_message update_progress 30`

Cq-Depend: chromium:2546329
Change-Id: I41d7d584b900ebad3f11536f40c6f33d6efe6be3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/chromiumos-assets/+/2546330
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>
Tested-by: Ting Shen <phoenixshen@chromium.org>
Commit-Queue: Ting Shen <phoenixshen@chromium.org>
diff --git a/images/boot_message.png b/images/boot_message.png
index ecc248e..42ea5fd 100644
--- a/images/boot_message.png
+++ b/images/boot_message.png
Binary files differ
diff --git a/images/boot_message_light.png b/images/boot_message_light.png
new file mode 100644
index 0000000..ecc248e
--- /dev/null
+++ b/images/boot_message_light.png
Binary files differ