Rename methods for creating and destroying user's storage keys

Users have many cryptographic keys, such as:

- Storage (file-based encryption) keys, both CE and DE
- Keystore super keys, both AfterFirstUnlock and UnlockedDeviceRequired
- Keystore client keys
- Synthetic password and the Keystore key that encrypts it

Unfortunately, many StorageManager, vold, and LockSettingsService
methods refer simply to "UserKey".  It isn't clear or consistent which
key(s) they mean.  For example, unlockUserKey unlocks CE storage,
createUserKey creates both the CE and DE keys, and
unlockUserKeyIfUnsecured unlocks CE storage but also does other things.
With the planned fixes to handle Keystore super keys more similarly to
the CE storage key (b/296464083), this confusion will increase further.

This CL is the second part of fixing this.  It renames the following
methods of StorageManager to make it clearer what they do:

   createUserKey => createUserStorageKeys
   destroyUserKey => destroyUserStorageKeys

No change in behavior, except a changed trace tag name and log message.

Bug: 306204742
Flag: exempt, mechanical refactoring
Test: presubmit
Change-Id: I5894beb97823dced5954e405d779fada49c79e8d