Enforce that internal storage is prepared first

Before doing anything else in fscrypt_prepare_user_storage(), error out
if adoptable storage is being prepared before internal storage.  Without
this explicit check, making this mistake results in a sequence of weird
errors that is hard to trace back to the actual problem.

Bug: 231387956
Change-Id: Ib26cc1bd46ffa2578f6f0156dfacc5496dae3178