[ci] Fix and standardize target/task names

[stuartmorgan]

Fixes the target names in LUCI to not use '+', since it's not supported.

To better align the names between the two different infrastructures,
also updates the task naming in cirrus.yml to not use +. It also
standardizes the naming form across both systems as:
<Host platform> <target platform>-<test_name> <extra info>
where:

• <Host platform> is always there on LUCI, where required, but only
  added where it's potentially ambiguous on Cirrus
• `' is omitted when the test is not target-platform
  specific
• `<test_name>' uses underscores, which is consistent with
  flutter/flutter (and with Cirrus step naming).
• <extra info> is only explicitly set (to the channel) on LUCI; Cirrus
  automatically adds channel info there due to the way matrix works.

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 relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.