UserController: avoid misleading log messages about failed to find key

If a user's CE storage requires a credential token to unlock, then
there's no need to try to unlock it without one.

This avoids logging some misleading error messages on every boot, like:

    E vold    : Failed to read from /data/misc/vold/user_keys/ce/0/current/keymaster_key_blob: No such file or directory
    E vold    : Failed to find working ce key for user 0
    E vold    : Couldn't read key for 0

Bug: 146206679

Test: atest com.android.server.am.UserControllerTest
      atest com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testResetPasswordWithToken

      Also a manual test: booted device that has a PIN.  Verified that
      the misleading log messages are no longer present.  Unlocked
      device, then removed the PIN and rebooted.  Verified that the
      device was then automatically unlocked as expected.

Change-Id: I5652a417727553f10a39eb97fc04cc5116b81347