Make setLockCredential() use doVerifyCredential()

Make setLockCredential() call the internal method doVerifyCredential()
instead of the public method verifyCredential().  This follows the best
practice for service implementations by not re-entering the Binder
interface as part of the implementation of a different method in the
interface.  Specifically, it avoids a redundant permission check that is
needed only for incoming Binder calls.  (setLockCredential() already
does its own permission check and clears the calling identity.)  It also
avoids a scheduleGc(), which again is done by setLockCredential().

No functional change other than removing redundant work.

Test: atest FrameworksServicesTests:com.android.server.locksettings
Bug: 320392352
Bug: 416768837
Flag: EXEMPT refactor
Change-Id: Iced66ee21db3c130f5606eb28d1f1b1d8385b836