Corrupt the PersistentDataBlock payload explicitly in factory reset

The current formatPartitionLocked only sets the data length to zero and
leave the payload unchanged. If the client side ignores the header and
read the payload directly, it still has the chance to read old data.
This CL corrupts the payload explicitly in formatPartitionLocked.

Bug: 185369514
Test: \
  1. Flash device, wipe
  2. Go through SUW with a google account. Set a PIN.
  3. Factory Reset through Settings.
  4. In SUW, when setting up an account (or copy from an old device),
     it should not ask for the previous PIN.

Change-Id: Ib60c68e3e91a4f0a7fd4e86b462091995bbe4fe2