Move checkNotSealed() to addBuilder()

Move checkNotSealed() to addBuilder()

addBuilder() is called after previous check for seal from all callers,
who do not have any side-effects aside allocating arguments to
addBuilder().

Move the call to checkNotSealed() to addBuilder(), making the code
more straightforward and possibly preventing bugs from creeping in.

JIRA: CONTROLLER-1887
Change-Id: I127385378957deb7f38f1203870a0e52cd630644
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 35fe9a2bb63d451d820707f5142349539334a7ad)