Adds NewAppLayoutEnabled Feature Flag

[ericdecanini]

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

Adds new_app_layout_enabled Feature Flag

Motivation and context

Closes #6578

We will use this feature flag to hide changes related to the Space Switching and Edit Layout projects until they are both ready to be released together

The flag can be found in debug menu > private settings

Screenshots / GIFs

Tests

• Go to the debug menu (either inside hamburger menu or first screen on auth flow)
• Go to features
• Enable feature flag
• Restart app and see feature flag is still enabled

Tested devices

• Physical
• Emulator
• OS version(s): Android 12

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]

I think in this case you should rather add a new VectorFeatures, rather than a new VectorOverrides. VectorOverrides should probably be removed, everything could be VectorFeatures. It does not block this PR to be merge though.
Let's discuss this with @ouchadam later.

[fedrunov]

LGTM

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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