Fix hover effect on open_by_default_button

This change updates the open_by_default_button's size, padding, and
background to ensure the hover effect matches other buttons.

Video: http://recall/-/dvR0xNyCWFr2PAfPfPhsQ6/f9pbqmEkOvCfniPyNoxnsn
Bug: 403072612
Flag: com.android.window.flags.enable_desktop_windowing_mode
Test: manually check the hover
Change-Id: I54746eb6d086909372e7beaedf2327a69f224b58