Securely encrypt the master key

Remove unencrypted link since it is easier to manage directly
Move creation of key to vold
Start vold early so this is possible in a timely fashion

This is one of four changes to enable this functionality:
  https://android-review.googlesource.com/#/c/144586/
  https://android-review.googlesource.com/#/c/144663/
  https://android-review.googlesource.com/#/c/144672/
  https://android-review.googlesource.com/#/c/144673/

Bug: 18151196
Change-Id: Idb17d1f1a724c6ec509d181ae4427113e9d3b5e6