Add a flag controlling API changes for Private Space

This change adds a flag to control only the API changes that are
required for enabling private space on the device. Currently, the
android.os.allow_private_profile flag controls both the API changes and
the client side changes to enable private space. With this change, we
intend to split the changes controlled by the allow_private_profile flag
into two parts:
- The API changes that would be controlled by the new flag
  allow_private_profile_apis
- Other changes to consume this new API, controlled by both
  allow_private_profile_apis and allow_private_profile flag

Test: m all_aconfig_declarations
Bug: 299069460
Change-Id: I4246a8f76e02028193abb466d572e4f84f8cb66c