Move {create,destroy}UserStorageKeys() to StorageManagerInternal

Move these methods from IStorageManager to StorageManagerInternal, since
they are used only within system_server itself.

Bug: 434738842
Flag: EXEMPT refactor
Test: atest FrameworksMockingServicesTests:StorageManagerServiceTest
Test: atest FrameworksMockingServicesTests:UserManagerServiceMockedTest
Test: atest FrameworksServicesTests:UserManagerServiceTest
Change-Id: I356d89a1ce5ae876b1439273be07ec144492ca90
6 files changed