For Android visual tests, they should be run with the right
device image (currently a Nexus 5X API 30). However,
there's no easy way to check the device type for Appium.
The check that was there before works on CI
since the DEVICE_SKIN environment variable is set there
(set by the android.cake script). And it can work locally if
folks run android.cake then launch VS from that console
session to get the env variable - but it doesn't work if VS
is run separately (normal case).

So update this with a different check. Getting the actual
Android device type requires using adb. We might eventually
do that, but for now just use a screen size check as a proxy
for the device type. Also update the failure message to give
good instructions.

uitest-prepare will build the test app, launch
the emulator, and deploy the test app. But
it won't actually run any tests.

Also display the Android image name