SyntheticPasswordCrypto: clean up AES-GCM constants

SyntheticPasswordCrypto.PROFILE_KEY_IV_SIZE was apparently copied from
LockSettingsService.PROFILE_KEY_IV_SIZE, which is the IV size used when
encrypting profile passwords.  But in SyntheticPasswordCrypto it has
nothing to do with profiles.  Rename it to AES_GCM_IV_SIZE.

For consistency, also rename AES_KEY_LENGTH to AES_GCM_KEY_SIZE and
DEFAULT_TAG_LENGTH_BITS to AES_GCM_TAG_SIZE, and change the latter from
bits to bytes.

No change in behavior; this is just a cleanup.

Test: atest com.android.server.locksettings (on Cuttlefish)
Change-Id: I3358826a7299b62e5be46682291bfc02733c8b1f