[modules][system-ui] Persist user selected background color (#22773)

# Why
Closes ENG-8736

# How
Setting the background color is now persisted and restored when the app
is launched (on iOS). O

[modules][system-ui] Persist user selected background color (#22773)

# Why
Closes ENG-8736

# How
Setting the background color is now persisted and restored when the app
is launched (on iOS). On Android, a new function had to be added to
restore the color on app start. If no color is provided the default will
be black when in dark mode or white otherwise.

# Test Plan
Tested on physical devices and emulators with bare expo.

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