Make Linux apps non-unique by default.

[robert-ancell]

Make Linux apps non-unique by default.

This stops them attempting to own a D-Bus name, which may not be possible if the
app is sandboxed (e.g. in a Snap). It's also probably what most developers would
expect is the correct behaviour by default.

https://forum.snapcraft.io/t/flutter-snapcraft-yaml/22150/7

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[stuartmorgan]

RSLGTM