crypto: Use OnlySigned isolation flag to setup decryption trust req.

[BillCarsonFr]

Depends on matrix-org/matrix-rust-sdk#4046

part of https://github.com/element-hq/crypto-internal/issues/353
For now the new decryption errors will show as regular ones (waiting for this message), a following PR will fix that once the sdk has support for different UTD errors

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• Changes have been tested on an Android device or Android emulator with API 23
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.68%. Comparing base (98d9abe) to head (3259539).
Report is 21 commits behind head on develop.

Additional details and impacted files

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##           develop    #3569   +/-   ##
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  Coverage    82.68%   82.68%           
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  Files         1731     1731           
  Lines        41014    41014           
  Branches      4973     4973           
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  Hits         33912    33912           
  Misses        5342     5342           
  Partials      1760     1760           

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[bmarty]

OK for me

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud