Move {prepare,destroy}UserStorage() to StorageManagerInternal

Move these methods from IStorageManager to StorageManagerInternal, since
they are used only within system_server itself.  Also add type
annotations and javadoc.

Bug: 434738842
Flag: EXEMPT refactor
Test: atest FrameworksMockingServicesTests:StorageManagerServiceTest
Test: atest PackageManagerServiceServerTests:UserDataPreparerTest
Change-Id: If6e09a551ecc620e6c877dc2971f6b8eb7da9db8