[starnix] Refactor the Starnix crypt service

I am hoping the crypt service does not need to be a singleton; multiple
remote volumes would not work correctly prior to this CL.

This moves a bunch of code into the Starnix crypt service which should
make it easier for some upcoming changs of mine.

Bug: 452741473
Change-Id: I6443eeef55d7d112f4f83b5b6fff7a365b5137bd
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1433554
Reviewed-by: Brandon Castellano <bcastell@google.com>
Reviewed-by: Maria Glukhova <mariagl@google.com>
Commit-Queue: Chris Suter <csuter@google.com>
Fuchsia-Auto-Submit: Chris Suter <csuter@google.com>