fix(android): remove "--" from XML comments in colors.xml

XML forbids "--" inside a comment, and both comments referenced the CSS
custom property by its "--background" name, so mergeDebugResources
failed to parse them.

android/android/app/src/main/res/values-night/colors.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
-    <!-- Matches the web app's dark --background (app.css). The app ships a
+    <!-- Matches the dark background custom property in web/src/app.css. The
-         full dark theme; a light splash flashed white before the WebView
+         app ships a full dark theme; a light splash flashed white before the
-         painted. -->
+         WebView painted. -->
     <color name="splash_background">#050813</color>
 </resources>

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

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
-    <!-- Matches the web app's light --background (app.css). -->
+    <!-- Matches the light background custom property in web/src/app.css. -->
     <color name="splash_background">#FFFBEB</color>
 </resources>