Emulator: ensure -> reset workflow is inefficient

[baltpeter]

If you want to use an emulator, you would typically first call ensureDevice() and then resetDevice(), just like we do in the example:

appstraction/examples/android-emulator.ts

Lines 17 to 18 in 63a1c8c

	await android.ensureDevice();
	await android.resetDevice(process.argv[4] || 'your-snapshot');

However, that is quite inefficient. With #27, we're starting to do quite a bit of prep work in ensureDevice(), like installing WireGuard. It's pretty silly to do that only to then immediately reset the emulator to a snapshot (which itself will call ensureDevice()).

But you can't just skip the first ensureDevice() call either since we do want to assert that the device is ready. Maybe we want to add a prepare: boolean parameter that defaults to true?