Read-only streams should not throw on Flush, adjust NotImplemented on Pickers

[MartinZikmund]

GitHub Issue (If applicable): fixes #5751, fixes #5766

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

• Some read-only streams in Storage APIs throw on Flush.
• Analyzers report file pickers as not implemented even though they are

What is the new behavior?

• Some of the read-only streams in Storage APIs no longer throw on Flush calls to match UWP behavior.
• Adjust NotImplemented attributes on file pickers

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

[gitpod-io]