Closes #1315: Re-introduce FxA interceptor to make auth work

This was removed accidentally, let's add it back in, but now with a clarifying comment :)

app/src/main/java/org/mozilla/fenix/AppRequestInterceptor.kt

@@ -9,8 +9,14 @@ import mozilla.components.browser.errorpages.ErrorPages
 import mozilla.components.browser.errorpages.ErrorType
 import mozilla.components.concept.engine.EngineSession
 import mozilla.components.concept.engine.request.RequestInterceptor
+import org.mozilla.fenix.ext.components
 
 class AppRequestInterceptor(private val context: Context) : RequestInterceptor {
+    override fun onLoadRequest(session: EngineSession, uri: String): RequestInterceptor.InterceptionResponse? {
+        // Accounts uses interception to check for a "success URL" in the sign-in flow to finalize authentication.
+        return context.components.services.accountsAuthFeature.interceptor.onLoadRequest(session, uri)
+    }
+
     override fun onErrorRequest(
         session: EngineSession,
         errorType: ErrorType,