Fix downgrades on device with FRP enabled

Unfortunately, non-forwards-compatible changes to the PasswordData
format break setting up an older version of Android on a device that had
Factory Reset Protection (FRP) set up on a newer version.

Commit 23070d87 ("Added changes for storing PIN length in
PasswordData") (http://ag/21983004) made a non-forwards-compatible
change to the PasswordData format by reusing the first two bytes of the
credentialType field as a version number.

Therefore, undo that part of the change, and just use the data length to
determine whether the new pinLength field is present or not.

Bug: 276780938
Test: atest com.android.server.locksettings
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:2353f5a9bc31cdef9c4d5329bced9bb6c79fd14d)
Merged-In: Ibf3c91d14a0c6bd9af4403b080532f2739fde119
Change-Id: Ibf3c91d14a0c6bd9af4403b080532f2739fde119