Use isCeStorageUnlocked() in ConfirmDeviceCredentialActivity

isUserKeyUnlocked() is being renamed to isCeStorageUnlocked() to make it
clear what it does (considering that there are many types of user keys).
Temporarily, the method exists under both names.  Change
ConfirmDeviceCredentialActivity to use the new name.  No change in
behavior.

Bug: 306204742
Flag: exempt, mechanical refactoring
Test: presubmit
Change-Id: I9a3f686b57cfbf99b6c915565e5ecc38ddfe9b22