Don't compress apexes produced by apex_test module

AFAIU, apex_test is used by modules to create a test version of their
apex for e2e testing of the update flow. Since it's a regular pattern
for test_apex to use the same apex_defaults as the "production" apex,
and due to the fact that we are only going to compress pre-installed
version of the apex, it makes sense to explicitly not compress
test apexes.

Bug: 185082717
Test: build test_com.android.media verified it's not compressed
Test: atest sdkextensions_e2e_tests
Change-Id: I1179235efe068fefc582369a4218ba967e7efbf3