Theme: Fix - Replace null with unspecified to standardize system default handling

[cmgustavo]

RN-2674

Changes

• Replaces all usages of null as the "system default" color scheme value with 'unspecified', aligning with the updated ColorSchemeName type in React Native ('light' | 'dark' | 'unspecified')
• Updates initial state, resetAllSettings, wallet import migration, the Theme screen UI, analytics/log labels, and the appearance change listener
• Adds backward-compatible null handling at all read sites (Root.tsx, index.js, Theme.tsx) so existing users with null persisted in storage continue to get correct system-default behavior after upgrading