Merge background.js and sw.js into bg.js

Now that we have included webpack-preprocessor-loader into Webpack, we
can merge the two files and hide code behind conditional statements
which will be preprocessed at compile time.

Change-Id: I99c87ca33fcb231d04c51ff9651e0cf10443e60b
diff --git a/templates/manifest.gjson b/templates/manifest.gjson
index 20999d8..2faf7fb 100644
--- a/templates/manifest.gjson
+++ b/templates/manifest.gjson
@@ -103,10 +103,10 @@
 #if defined(CHROMIUM)
     "persistent": false,
 #endif
-    "scripts": ["background.bundle.js"]
+    "scripts": ["bg.bundle.js"]
 #endif
 #if defined(CHROMIUM_MV3)
-    "service_worker": "sw.bundle.js"
+    "service_worker": "bg.bundle.js"
 #endif
   },
 #if defined(GECKO)