Use a keymaster-based key storage module

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
6 files changed