Support auto-launching emulators from launch.json #5520

If you have a launch config with a hard-coded device ID (possibly using #5516), it'll only work if the emulator device exists. We should support an additional field emulatorId that can be used to auto-launch an emulator and use that (by looking up the device with that emulator ID).

(we'd need a second field because emulator and device IDs are not the same)