Fix search box icon baseline focus outline disappearing on hover
Previously, search_box_icon_background_selector_baseline.xml used a
single StateListDrawable (<selector>) to manage all interaction states.
Because the state_hovered item was defined above the state_focused item,
hovering over an already-focused search box button would cause the
selector to match the hovered state first, replacing the focus outline
ring with the solid hover background.
This CL fixes the issue by wrapping the selectors in a <layer-list>.
The solid background overlays (hovered, pressed) are now handled
independently on a lower layer, while the focus ring outline is
handled on a higher layer. This ensures the focus ring remains
visible even when the icon is simultaneously being hovered or pressed.