Fix oauth sqlite migration downgrade failure

The oauth migration downgrade was failing because it tried to drop
a constraint which sqlite doesn't support. The code tried to not do
the drop on sqlite but didn't check for the sqlite engine properly.

Change-Id: Ie7900d352a26900f913bdefe1eb93d511283efc0
Closes-Bug: #1366606
Merge "Fix oauth sqlite migration downgrade failure"