Hide the Manually Verify by Text option behind devtool flag

[BillCarsonFr]

The manually verify by text option is an advanced feature, that will mark a device as verified for everyone by by-passing the real verification process.
As a result the targeted session will be verified but will not have access to other secrets (not be able to decrypt history, not capable of verifying new devices, ..).

Web as moved this as a dev tool too (element-hq/element-web#30094).

We have reports of users accidentaly using this advanced feature, resulting in half setup devices.
It will now be shown anymore unless the developper mode is enabled.

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

Motivation and context

Screenshots / GIFs

Tests

• Step 1
• Step 2
• Step ...

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/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()