[Poll] Improve rendering of poll end message when poll start event isn't available (PSG-1157)

[mnaturel]

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

Add a special rendering for the message when receiving a poll End event while the related poll start is not available in the timeline.
Send a fallback text in the poll end event (may be needed by other Matrix clients).

Motivation and context

Closes #8129

Screenshots / GIFs

Tests

• Create a new room
• Set the setting in the room so that new joiners cannot see the history
• Create a poll
• Invite another user
• Wait for the user to join
• End the previously created poll
• Check the other user sees a simple "Poll ended" message without any other info

Tested devices

• Physical
• Emulator
• OS version(s): Android 11

Checklist

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

8.3% Coverage
0.0% Duplication

[yostyle]

LGTM