Add setting to disable URL/email specific layout

[MiMoHo]

What

Adds a preference "Adapt layout for URL and email fields" (default on, i.e. current behaviour). When turned off, getKeyboardMode() returns TEXT for URI and email input types, so the base layout and popups stay identical to a normal text field: the comma key stays a comma (no / or @ swap), and no TLD popups are shown.

The whole change is a single gate in KeyboardLayoutSet.getKeyboardMode():

InputType.TYPE_CLASS_TEXT ->
    if (!Settings.getValues().mUrlEmailLayout) KeyboardMode.TEXT
    else if (InputTypeUtils.isEmailVariation(variation)) KeyboardMode.EMAIL
    else if (variation == InputType.TYPE_TEXT_VARIATION_URI) KeyboardMode.URL
    else KeyboardMode.TEXT

plus the pref plumbing (Settings/Defaults/SettingsValues) and one SwitchPreference in the Preferences screen, next to "Show TLD popup keys".

Why

In URL fields the comma key becomes / and in email fields @ (the variation_selector in functional_keys.json). This is intentional and can already be reverted with a custom functional-keys layout, as pointed out in #1573 and #734.

However, some users specifically want a keyboard whose base layout never shifts depending on the field — muscle memory for the comma position being the main reason — and building a full custom functional-keys layout is a fairly heavy step for "please don't move my comma". This offers a one-switch opt-out for that preference, without changing anything for everyone else.

Notes / open questions

• Draft, opening this mainly for a decision: is a built-in setting wanted here, or should this stay "customize your layout"? Happy to close if the latter.
• Default is unchanged behaviour → existing users see no difference.
• Scope choice: the switch disables the entire URL/email mode (base key and TLD popups), which matches "no automatic adaptation at all". If you'd rather it only kept the comma/period base keys but still showed TLD popups, I can narrow it to the variation_selector instead.
• English strings only; no translations touched.
• Builds cleanly (:app:compileDebugSources).

Refs #1573, #734.