Make sure disconnected devices / apps can be imported and exported

Summary:
It should be possible to exported disconnected devices, so that flipper traces / support form reports can be created from them. This diff introduces this functionality. Support for plugins with custom export logic is introduced in a later diff.

Issues fixed in this diff:
- don't try to take a screenshot for a disconnected device (this would hang forever)
- device plugins were always exported, regardless whether the user did select them or not
- sandy plugins were never part of exported disconnected clients
- increased the amount of data exported for device logs to ~10 MB. This makes more sense now as the logs will no longer be included in all cases
- fixed issue where are plugins would appear to be enabled after the client disconnected (this bug is the result of some unfortunate naming of `isArchived` vs `isConnected` semantics. Will clean up those names in a later diff.

Changelog: It is now possible to create a Flipper trace for disconnected devices and apps

Reviewed By: nikoant

Differential Revision: D26250894

fbshipit-source-id: 4dd0ec0cb152b1a8f649c31913e80efc25bcc5dd

desktop/flipper-plugin/src/plugin/Plugin.tsx

@@ -276,7 +276,13 @@ export class SandyPluginInstance extends BasePluginInstance {
 
   private assertConnected() {
     this.assertNotDestroyed();
-    if (!this.connected.get()) {
+    if (
+      // This is a better-safe-than-sorry; just the first condition should suffice
+      !this.connected.get() ||
+      !this.realClient.connected.get() ||
+      !this.device.isConnected ||
+      this.device.isArchived
+    ) {
       throw new Error('Plugin is not connected');
     }
   }