Add a safelyBlurElement util.

[tugorez]

The safelyBlurElement method in the view manager should become the standard way to "blur" and "remove" elements within the web engine. Calling this method ensures the blur operation doesn't disrupt the framework's view focus management.

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I signed the CLA.
• [x] All existing and new tests are passing.

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