[Linux][A11y] use ATK_ROLE_PASSWORD_TEXT for obscured text

[jpnurmi]

This makes TextField(obscureText: true) report its accessiblity role correctly:

Before	After

Fixes a small part of flutter/flutter#103191.

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] and the [C++, Objective-C, Java style guides].
• 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.

[jpnurmi]

@robert-ancell, not sure if you're watching Linux-related engine PRs but here's one. :)

[robert-ancell]

LGTM