Add ConfigStore to use feature flags in DocsUI.

Created ConfigStore class to provide feature flags from device config to
be used in DocsUI. This also allows parametrizing the existing tests for
the private space feature flag.
Added check for S+ for using the feature flag in production as well as
testing.

Bug: 321894925
Test: atest unit tests and functional tests
Change-Id: I6cc8297d5a1dc5f0dfe850485320b38232f006fb