Add an additional flag guard for Private Space implementation

[SettingsLib & SystemUI]

This a layer of flag guarding only for the implementation of Private Space features excluding the APIs. The MVP flag allow_private_profile still guards all the features including the APIs.

Bug: 326060689
Test: Manual - a few verifications that some feeatures are unavailable when this flag is disabled
Test: Run presubmits and verify that nothing breaks

Change-Id: I61acf8fe98756cb1055fa4f554fc37517c5342da