Implement global Gemini Code Assist for individuals Privacy setting

### What would you like to be added?

Implement a global (per google account) privacy setting for "Gemini Code Assist for individuals".

Currently, in us-docker.pkg.dev/gemini-code-dev/gemini-cli/sandbox:0.32.1, the selection whether to `Allow Google to use this data to develop and improve our products?` seems not persisting. If I start a new container of gemini-cli, I need to configure `/privacy` again.

Alternatively, provide a setting for both settings.json and as an environment variable that controls the `/privacy` behavior.

### Why is this needed?

To make possible running gemini-cli in an unattended way, while setting the desired privacy.

### Additional context

Compare this to how for Anthropic Claude Code, the privacy settings are set on the account level, not per tool instance https://github.com/google-gemini/gemini-cli/issues/14104#issuecomment-3795489165