[rotang] Fix panic when fetching legacy calendar credentials.

The problem here is another one of those context.Context mismatches.
In this project there are 3, at least, contexts.

- ^router.Context - Used by the luci/gae handlers and packages.
- context.Context - The normal context used by most normal appengine packages.
- appengine context.Context - For when using cloud APIs like Calendar and email.

When a Cloud API is presented with the wrong kind of context the service panics.

To clean this up the credentials functions now take a *router.Context instead of
just a context.Context. This b/c the legacy credential function use both the normal
context (datastore) and the appengine conext (calendar/backup). The appeninge context
can be created from the *router.Context.Request.

Bug: 927559
Change-Id: Ic4dec8c4fcdb31bc3600948efa97d21ec6672b15
Reviewed-on: https://chromium-review.googlesource.com/c/1449352
Reviewed-by: Scott Lewis-Kelly <slewiskelly@google.com>
Commit-Queue: Ola Karlsson <olakar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#20437}