Remove isSyntheticPasswordBasedCredentialLocked()

As an additional cleanup, now that all users are guaranteed to have a
synthetic password (except for new users during early boot), remove the
isSyntheticPasswordBasedCredentialLocked() method and its callers.

Considering each case:

- In migrateFrpCredential(), the user is guaranteed to have an SP, since
  they also have an LSKF.  This was true even before "SPs on creation".

- In onThirdPartyAppsStarted(), any users in mEarlyCreatedUsers are
  guaranteed to not have an SP yet, since their SP creation was delayed,
  and the code that did on-demand SP creation has been removed (as it
  should not have been reachable anyway).

- In getCredentialTypeInternal(), the LSKF-based protector ID is being
  looked up anyway.  It's more efficient to check that value for
  NULL_PROTECTOR_ID, instead of doing a redundant lookup.

- In doVerifyCredential(), the check for an SP was redundant with later
  checks.  So, removing it doesn't change the behavior (other than the
  log messages); VerifyCredentialResponse.ERROR is still returned.
  Also, the SP should always exist here anyway.

- Similarly, in getHashFactor(), the check for an SP is redundant with
  the check for NULL_PROTECTOR_ID in unlockLskfBasedProtector().

- In disableEscrowTokenOnNonManagedDevicesIfNeeded(), calling
  destroyEscrowData() is harmless if there is no SP.  But there should
  always be an SP here anyway.

Test: atest com.android.server.locksettings
Bug: 232452368
Change-Id: I39ad1bdf84db745db85d4d8fcaaa1d989511d0e1