Revert "Always apply compat changes to system apps."

This reverts commit 345b1d53.

Reason for revert: We should respect the targetSDK of system apps as
well, and there's no reason not to support them as far as I can see.
Usecase example: ag/9025572

Bug: 135010838
Test: atest CompatConfigTest
Change-Id: I256c377e4edc59ba998609f4d3b6b87edeb8fff3