[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
(cherry picked from commit ceeca68b)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:3a04799f68de6b77fc89faddda032b1031551e78)
Merged-In: Ie281bbdc8788ee64ff99a7c5150da7ce7926235e
Change-Id: Ie281bbdc8788ee64ff99a7c5150da7ce7926235e