Android Settings can delete Leo AI history

[petemill]

Android needs some settings update to reach parity with Desktop and be allowed to enable persistant storage for Leo by default:

• Browser Data deletion gets Leo AI checkbox which deletes all conversations in the provided timeframe.
• The "Leo" section of preference's "reset data" button now deletes all conversations (and has its wording changed to match desktop).

These were previously implemented for desktop in:

• AIChat conversation data storage brave-core#25876
• AI Chat conversations should persist browser restart #42800

Related:

• AI Chat history feature should be disabled by default on Android #42928

Test Plan

On:

• Adds preferences and methods to delete Leo conversations on Android brave-core#27074

[srirambv]

Verification passed on Google Pixel 8 with Android 15 running 1.75.171 x64 build

• Verified steps from brave/brave-core#27074
• Verified selecting Settings -> Leo -> Delete all Leo AI conversation data deletes all AI conversation data
• Verified with deleting all browser data including Leo AI all AI conversation is delted
• Verified visiting a site and summarising it and deleting browsing data, going back to Leo doesn't show the title or logo

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