vold: cryptfs: Don't update KDF without validating pwd/key.

Prior to this, the Key derivation function would get
 blindly updated even if the user entered the wrong password.
Now, we only attempt to upgrade the KDF if the pwd/key have
been verified (i.e. after a successful mount).

Bug: 11460197
Change-Id: I0469228cc9b87c47754e8ca3c7146651da177da5
vold: cryptfs: Don't update KDF without validating pwd/key.

Prior to this, the Key derivation function would get
 blindly updated even if the user entered the wrong password.
Now, we only attempt to upgrade the KDF if the pwd/key have
been verified (i.e. after a successful mount).

Bug: 11460197
Change-Id: I0469228cc9b87c47754e8ca3c7146651da177da5