Color Picker Fragment Dark Mode Toggle (1/2)

Move dark mode toggle from the customization hub landing page into the
color picker page, gated by revamped UI flag. Currently the dark mode
section controller is kept and used in the color picker page, but this
can be refactored in the future.

Bug: 269449457
Test: Manual
Change-Id: I2669cee07fa81fcd8c436a30c511bae66aa50a29