Move the notification part out of ConsumerEnrollmentHandler so that it can be reused for unenrollment as well.
ConsumerManagementNotification is always created for owner profiles even when
there is no pending notification to show, because enrollment or unenrollment
may happen after the owner signs in.
BUG=
chromium:353050
TEST=unit_tests
Committed:
https://crrev.com/fcbf1a950801608326e710c50f4427b9da5fac9c
Cr-Commit-Position: refs/heads/master@{#304998}