Add the warning and configuration on first launch

[k3b]

This is a followup to #2 and to the "include into fdroid discussion" in https://gitlab.com/fdroid/fdroiddata/-/merge_requests/10680

"warning" means the search engines collect userdata about people using their service. Example: In germany i cannot use google to search if my pc/phone is not connected to a gmail account.

Currently all available searchengines are enabled by default and can be disabled in the settings (opt out).

For privacy reasons the fdroid community prefers a opt in.

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Proposed solution:

• On app Startup show the settings-page if none of the searchengines are selected.

• Modify the Settings-page to contain a static text:

  • Privacy Warning:
  • This app uses commercial searchengines that may store and use your data and uploaded image for other purposes.

• replace MultiSelectListPreference with 3 separate CheckBoxPreference so that there is no more need to click on the "search"-setting to see the engine configuration.

[k3b]

implemented in https://github.com/OHermesJunior/imagesearcher/pull/11