[components] Closes mozilla-mobile/android-components#874: browser-en…

…gine-system: Rename strings file.

Background: We are going to reserve the name "strings.xml" for translatable strings. This avoids that
our L10N tools need to be able to parse and process all kinds of resources that are not strings.

X-Channel-Revision: [main] mozilla-mobile/android-components@cf522a7
X-Channel-Revision: [main] mozilla-mobile/fenix@20b25d7
X-Channel-Revision: [main] mozilla-mobile/focus-android@29486b8
X-Channel-Converted-Revision: [ac-prep] mozilla-mobile/firefox-android@ff58c45

_meta/mozilla-mobile-firefox-android.json

@@ -1,6 +1,6 @@
 {
   "name": "mozilla-mobile/firefox-android",
   "revs": {
-    "ac-prep": "c56df56464ac697dd25489b5f69fbc53fc39db58"
+    "ac-prep": "ff58c451ca6ee08f4d4d21853bb61538202bbd40"
   }
 }