Fixes the hover color of app header buttons

Previously, header buttons used RippleDrawable for their background.
This prevented setting a specific hover background color, as
RippleDrawable automatically uses a 20% alpha version of the ripple
color for hover effects.

This change replaces RippleDrawable with ShapeDrawable, which allows the
desired hover background color to be set.

Video: http://recall/-/dvR0xNyCWFr2PAfPfPhsQ6/dW9qniSJcukXo9lWR3Xquq
Test: None
Flag: com.android.window.flags.enable_desktop_windowing_mode
Bug: 365121303
Change-Id: I43e3ac4adc95956b49916355d297b8c78f3abc86