Preserve device state after disconnect

Summary:
This diff stack introduces support for keeping devices and clients around after they have disconnected. This is a pretty important debugging improvement, that will allow inspecting a device / app after it crashed for example.

This feature existed partially before, but only supported Android, and only support plugins with persisted state; as it replace the current device with an archived version of the same device. In practice this didn't work really well, as most plugins would not be available, and all non-persisted state would be lost.

This diff makes sure we can keep devices around after disconnecting, the next one will keep the clients around as well. And explain some code choices in more detail.

Note that `Device.isArchived` was an overloaded term before, and even more now (both representing imported and disconnected devices), will address that in a later diff.

https://github.com/facebook/flipper/issues/1460
https://github.com/facebook/flipper/issues/812
https://github.com/facebook/flipper/issues/1487

Changelog: iOS and Android devices will preserve their state after being disconnected

Reviewed By: nikoant

Differential Revision: D26224310

fbshipit-source-id: 7dfc93c2a109a51c2880ec212a00463bc8d32041

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

@@ -132,7 +132,9 @@ export abstract class BasePluginInstance {
       realDevice, // TODO: temporarily, clean up T70688226
       // N.B. we model OS as string, not as enum, to make custom device types possible in the future
       os: realDevice.os,
-      isArchived: realDevice.isArchived,
+      get isArchived() {
+        return realDevice.isArchived;
+      },
       deviceType: realDevice.deviceType,
 
       onLogEntry(cb) {