Drop ancient default salt value

A value for the salt is present in all code paths:

- All callers of `Password::derive_key` pass a `Some(..)` value for the
  salt.  Remove this `Some` everywhere and...
- Change the signature of `derive_key` to expect a `&[u8]` for `salt`
  rather than an `Option<&[u8]>`.
- `Password::derive_key` is the only caller of `generateKeyFromPassword`
  (via bindgen to C++-land), so...
- Make it clear that the C++ `generateKeyFromPassword()` function
  expects 16 bytes of `salt`.

Bug: 172121323
Test: compile, TreeHugger
Change-Id: I41966cce96b5371785680c13bacfc5f95969372a
6 files changed