Discover devices in parallel instead of serially waiting for each device type

[jmagman]

Description

For the next part of wireless iOS device support, make all the DeviceDiscovery.devices calls run at the same time in a Future.wait<List<Device>>(). This is necessary so the network device timeouts (to be introduced in the next PR) increase the time linearly instead of exponentially.

Breaking out just this change to make the next PR smaller, and changing the Stream=>List return value in the signature created some churn.

Related Issues

Another piece of #15072.

Tests

Updated tests to use lists instead of streams.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
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Breaking Change

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.

[jmagman]

@zanderso in #49854 (comment):

This could return a Stream if we're actually getting results bit-by-bit [...] or if this is implementing an interface that requires it to be a Stream, but otherwise we should probably prefer Future<List>, or Future<Iterable>.

(async* Streams also can have some surprising behavior. IIRC they don't interact well with FakeAsync for tests.)

[jonahwilliams]

This change looks good to me, but it will likely require a corresponding update in the google3 tool- I can point you at the sources tomorrow.

[jonahwilliams]

LGTM

[christopherfujino]

This is great, LGTM

[jmagman]

Kicking cocoon, landing on red.