Fix fallbackKeySharingStrategy

[abaker]

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

This PR fixes the fallback key sharing strategy to return the correct value

Motivation and context

The fallback key sharing strategy was always set to OutboundSessionKeySharingStrategy.WhenSendingEvent regardless of the config setting

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]

Nice catch, thanks!