Add scrolling clouds to CorgUI background.
Add scrolling clouds to CorgUI background.

Add cloud textures to CorgUI background to fill in empty space. Animate the
cloud textures to very slowly and leisurely move back and forth between two
positions, with the clouds always fully on screen.
PiperOrigin-RevId: 146164955
Change-Id: Idae22fb0c60278eb6a722741b02778df2b95446f

google/flatui,google/flatui,google/flatui,google/flatui