FlatButtonBackground: Fix background disappearing when pressing checkable ToolButton

Oliver Beardrequested to merge
work/olib/toolbutton-press into master

Pressed seems to go false, causing the animation to run to hide the background, and then checked becomes true immediately after, restoring the background. Using a NumberAnimation instead will interrupt itself and make no visible change.

Before/after:

Screencast_20241107_112546.webm

Replacing the animator does mean the animation is not run on the render thread, but this shouldn't be a problem as other behaviours on opacity in PC3 use NumberAnimation.

Potential candidate for 🍒.

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