fix backup looping same keys

[BillCarsonFr]

Type of change

• Bugfix

Content

Fixes a regression on backup introduced by #5853
The backedUp boolean was reset to false by the InboundGroupSessionStore when saving inbound session.
Also there is no point in persisting back inbound group session after a decryption, so I removed that.

Motivation and context

Tests

Added back the KeysBackupTest, they were ignored.

Tested devices

• Physical
• Emulator
• OS version(s):

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()

[bmarty]

LGTM. Detekt error can be ignored, the issue was on the develop branch.
Let's wait for the test to be fully run before merging this PR.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

88.9% Coverage
0.0% Duplication