vold: Clear disk encryption key on first boot after userdata wipe

If we detect userdata is unencrypted, and start encrypting it due to
forceencrypt, then also wipe hw encryption keys so we reset to default
password. Factory data reset usecase falls under this category.

Change-Id: I7b2c139e84b420f1dbc4e74e2f98f43fac3170d5
1 file changed