tests/android: Use adb reverse to access host webserver, allow running on !macos

[MarijnS95]

Instead of chaining a bunch of Unix commands to pry out the IP address of a random adapter, that may not be available on every distro, use the standard adb reverse command to forward a specific port from the Android phone (or emulator) to the host, where the test-webserver is running. This makes the test-script host-independent (should in theory run even on Windows now) and does not require the device at test (which could be a regular Android phone if not having an emulator running) to be connected to / reachable over the same network.

[MarijnS95]

Note that we could have also emitted this setup into the app's Cargo.toml like:

[package.metadata.android.reverse_port_forward]
"tcp:{}" = "tcp:{}"

As cargo-apk has native support for it, but calling the command ourselves is just as straightforward.

Note²: we could also use tcp:0 to let adb figure out a free port on the phone side, and route that to the webserver port (which need not necessarily be the same: that's why the tcp:{} argument is passed twice to adb reverse), see also the documentation:

 reverse [--no-rebind] REMOTE LOCAL
     reverse socket connection using:
       tcp:<port> (<remote> may be "tcp:0" to pick any open port)

[amodm]

As part of this, let's also then make ubuntu-latest as the default runner for android tests then, instead of macos-latest, as the latter runner is slightly slower (more contention probably) than the linux one.

You can make the change, or I can do it, assuming I have permissions to push on your branch.

[MarijnS95]

@amodm long ago it was pointed out that only MacOS GitHub runners supported hardware acceleration for the Android emulator, and not their Ubuntu machines for some (obscure, iirc) reason. Not sure if that's still the case, but I'd recommend doing it in a separate documented PR and comparing the timings with previous runs to be sure.

[amodm]

Yeah, you're right. A quick search doesn't seem to show up HAXM support on Linux. We'll continue to keep the runner on macos-latest for now.