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Instead of writing raw keys, encrypt the keys with keymaster. This
paves the way to protecting them with auth tokens and passwords later.
In addition, fold in the hash of a 16k file into their encryption, to
ensure secure deletion works properly.

Now even C++ier!

Bug: 22502684
Bug: 22950892
Change-Id: If70f139e342373533c42d5a298444b8438428322