[RESTRICT AUTOMERGE][pm] still allow debuggable for system app downgrades

Turns out we do have internal tests that downgrades system apps, so adding this exception to allow for that.

BUG: 267232653
BUG: 256202273

Test: manual
Change-Id: Ie281bbdc8788ee64ff99a7c5150da7ce7926235e
(cherry picked from commit ceeca68b)