Make 'sign in' flow more important in the initial screen

[jmartinesp]

Type of change

• Feature
• Bugfix
• Technical
• Other : design change.

Content

• "I already have an account" and "Create account" buttons in the initial screens have swaped places and styles.
• "I already have an account" is now "Sign in" to make it clearer to the user.

Motivation and context

Requested by a customer.

Screenshots / GIFs

The new design should look like:

Tests

• Open the app, check the buttons look like the design above.

Tested devices

• Physical
• Emulator
• OS version(s): 14

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/element-hq/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/element-hq/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 this is against all the other apps are doing.
I let @nadonomy give more input on this, the way it is done now was a strong requirement from the design team.

[jmartinesp]

@bmarty I think Nad already said he was ok with the changes in the associated issue.

[bmarty]

LGTM, thanks!