[ProgressIndicator Demo]Added label for progress indicator without percentage value

[harperl-lgtm]

This PR added semanticLabel to the Progress Indicator without a percentage value.

Issues fixed:
[b/239462156]

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 the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[guidezpl]

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