Skip private profile tests on unsupported devices

Here, we fix the private profile test by skipping them on unsupported
devices. Also, we move to using "RequiresFlagsEnabled" annotation
instead of mocking the feature flags for the read only flag
block_private_space_creation to fix the failures caused in
setFlagsEnabled.

Test: UserManagerServiceTest
Bug: 331238119

Change-Id: I11fa1b9991f8bc24f19f2b6c660d8a9ca07a370e