FFs can now be toggled in release builds too (#3101)

- Removed `StaticFeatureFlagProvider`.
- Always provide `PreferencesFeatureFlagProvider`.
- For the default values of feature flags, use a lambda with a `BuildMeta` parameter so we can customize the return value based on its data.

appconfig/src/main/kotlin/io/element/android/appconfig/OnBoardingConfig.kt

@@ -18,7 +18,7 @@ package io.element.android.appconfig
 
 object OnBoardingConfig {
     /** Whether the user can use QR code login. */
-    const val CAN_LOGIN_WITH_QR_CODE = false
+    const val CAN_LOGIN_WITH_QR_CODE = true
 
     /** Whether the user can create an account using the app. */
     const val CAN_CREATE_ACCOUNT = false