Remove userSerial param from vold methods that don't use it

createUserStorageKeys(), unlockCeStorage(), and prepareUserStorage()
have a user serial number parameter, but they don't actually do anything
with it except log it.  Remove this unnecessary parameter.

Bug: 316035110
Test: presubmit
Flag: N/A, mechanical refactoring
Change-Id: I73ebae1afb2bdb7ca856b40b34ce806fdda718fe