Media: Fix Image Editor buttons becoming unreachable via keyboard

[R1shabh-Gupta]

imageEdit.focusManager() unconditionally set tabindex="-1" on whichever element it moved focus to when the Image Editor dialog opened, either an admin notice or, when no notice was present, the first tabbable control in the dialog (such as the Crop button).

This is necessary for the admin notice because it is a <div> and is not natively focusable. However, native controls like <button> elements are already focusable and part of the normal Tab order. Adding tabindex="-1" removes them from that order, and the attribute was never removed afterward.

As a result, when the dialog opened without a notice, the initially focused control was permanently removed from the keyboard Tab order. Repeated open and close cycles progressively affected more controls (Crop, then Scale, then the rotation control, and so on).

This PR limits the tabindex="-1" assignment to the admin notice only. When no notice is present, focus is moved directly to the first tabbable control without modifying its tabindex, preserving the expected keyboard navigation.

Trac ticket: https://core.trac.wordpress.org/ticket/65814

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