Add a flag to hide / unhide the PS settings

As trunk stable feature flags are not fully available yet, this will
help prevent the PS settings availabe on droidfood builds by default.

Test: setprop sys.fflag.override.settings_private_space_settings
Test: manual from developer options
Bug: 295516544
Change-Id: Id8d134b661f43f775b750974c3e1ab8ecc03d0d6