Omit MetaMask extensionId from site metadata

The site metadata was updated in #7218 to include the extension id of
the extension connecting to MetaMask. This was done to allow external
extensions to connect with MetaMask, so that we could show the id on
the provider approval screen.

Unbeknownst to me at the time, the extension id was being set for all
connections to MetaMask from dapps. The id was set to MetaMask's id,
because the connections are made through MetaMask's contentscript.

This has been updated to only set the id when accepting a connection
from a different extension.

app/scripts/background.js

@@ -391,7 +391,10 @@ function setupController (initState, initLangCode) {
   // communication with page or other extension
   function connectExternal (remotePort) {
     const senderUrl = new URL(remotePort.sender.url)
-    const extensionId = remotePort.sender.id
+    let extensionId
+    if (remotePort.sender.id !== extension.runtime.id) {
+      extensionId = remotePort.sender.id
+    }
     const portStream = new PortStream(remotePort)
     controller.setupUntrustedCommunication(portStream, senderUrl, extensionId)
   }