KeyMint HAL: clarify spec text

 - Make clear that CERTIFICATE_NOT_{BEFORE,AFTER} must be specified for
   generating/importing asymmetric keys.
 - Fix enforcement level of Tag::UNLOCKED_DEVICE_REQUIRED.
 - Fix reference to exportKey() for Tag::STORAGE_KEY to mention
   convertStorageKeyToEphemeral instead.
 - Mark Tag::CONFIRMATION_TOKEN as deprecated.

Test: none, comment change
Bug: 188672564
Merged-In: I68727b024f6b6743403941763aefca64e3eb091a
Change-Id: I68727b024f6b6743403941763aefca64e3eb091a
Ignore-AOSP-First: already merged in aosp/master