Fix lightweight Picker traversal controls when non-traversable

[jsfan3]

Summary

• Hide lightweight Picker next/previous controls when the picker itself is not traversable.
• Prevent lightweight picker next/previous commands from moving focus out of a non-traversable picker.
• Keep the lightweight popup behavior aligned with Component#setTraversable(false) / preferred tab index traversal.

Problem

Picker defaults into the focus traversal order by setting a preferred tab index in its constructor. Applications can opt a picker out of traversal via setTraversable(false), which makes the component absent from the form's tab iterator.

The lightweight picker popup still calls Form#getTabIterator(Picker.this) to decide whether to show previous/next buttons. When the picker has been made non-traversable, the tab iterator cannot find it and initializes its current position to -1. In that state hasNext() may still return the first traversable component in the form, and hasPrevious() may return the last traversable component. As a result, the lightweight popup can display traversal controls even though the picker opted out of traversal.

Pressing one of those controls then calls getNextComponent(Picker.this) / getPreviousComponent(Picker.this), requests focus on an unrelated component, and starts editing it. On Android this can surface as an unexpected native text editor appearing for a later TextField immediately after the picker closes.

Fix

Gate both sides of the lightweight picker traversal behavior on Picker.this.isTraversable():

• the previous/next buttons are only created for traversable pickers;
• COMMAND_NEXT / COMMAND_PREV only attempt focus transfer for traversable pickers.

This preserves the existing traversal behavior for normal traversable pickers while respecting applications that explicitly call setTraversable(false).

Verification

• Reproduced in an Android app with a lightweight string Picker followed by numeric TextFields.
• Before this patch: picker.setTraversable(false) still shows the previous/next arrow buttons; pressing an arrow closes the picker and starts editing a numeric text field.
• With this patch: non-traversable lightweight pickers do not expose or execute previous/next traversal controls.
• Ran git diff --check on the branch.

[jsfan3]

@shai-almog I used generative AI to create this pull request, which addresses a real-world problem on my Android device. The fix seems reasonable and straightforward, and I don't think testing it locally by recompiling CN1 is necessary. However, if you would like me to do so, I can.