Add an additional flag guard for Private Space implementation

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: I05f7e2f20c6132b33484bb133ce03a933ece485f

src/com/android/settings/privatespace/PrivateSpaceAuthenticationActivity.java

@@ -77,7 +77,8 @@ public class PrivateSpaceAuthenticationActivity extends FragmentActivity {
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
 
-        if (Flags.allowPrivateProfile()) {
+        if (Flags.allowPrivateProfile()
+                && android.multiuser.Flags.enablePrivateSpaceFeatures()) {
             ThemeHelper.trySetDynamicColor(this);
             mPrivateSpaceMaintainer =
                     new Injector().injectPrivateSpaceMaintainer(getApplicationContext());