Fold read_and_install_user_ce_key() into fscrypt_unlock_user_key()

No change in behavior, except for removing a redundant check of
's_ce_policies.count(user_id)' and removing an extra ERROR message.

Test: see I7f11a135d8550618cd96013f834cebd54be5ef84
Change-Id: If221e23991e8e04138ae7dbdafe8160b00893655
Merged-In: If221e23991e8e04138ae7dbdafe8160b00893655
(cherry picked from commit 92428b24)