Avoid multiple instances of MessageActivity.

[Sagar0-0]

🎯 Goal

To avoid creating multiple instances of MessageActivity in the backstack when coming from PinnedMessage. It keeps the sample app simple and wont change the expected behavior.

🛠 Implementation details

When starting message activity, instead of creating a new instance, use the last one and pass the intent to onNewIntent.

🧪 Testing

Manually test.

☑️Contributor Checklist

General

• I have signed the Stream CLA (required)
• Assigned a person / code owner group (required)
• Thread with the PR link started in a respective Slack channel (#android-chat-core or #android-chat-ui) (required)
• PR is linked to the GitHub issue it resolves

Code & documentation

• Changelog is updated with client-facing changes
• New code is covered by unit tests
• Comparison screenshots added for visual changes
• Affected documentation updated (KDocs, docusaurus, tutorial)

☑️Reviewer Checklist

• UI Components sample runs & works
• Compose sample runs & works
• UI Changes correct (before & after images)
• Bugs validated (bugfixes)
• New feature tested and works
• Release notes and docs clearly describe changes
• All code we touched has new or updated KDocs
• Check the SDK Size Comparison table in the CI logs

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