Don't try to talk to vold mid-encryption.

We were attempting to unconditionally validate the encryption state on
CryptKeeper bringup, which required MountService to talk to vold. For
some reason, during encryption, this cannot happen, and that call never
returns, so the CryptKeeper UI was never brought up.

Bug: 5276690
Change-Id: I6a146e25e24f4efd760b0afa1e1409bf9ea3e9c3