apex_sepolicy_tests only if default payload type is ext4

apex_sepolicy_tests relies on `deapexer list -Z`, which is only
available to ext4 type.

Hence, when building with other type, it logs error messages like:

  erofs is not supported for `list`.

Until deapexer supports the feature, let's run the test for ext4.

Bug: 353896817
Test: PRODUCT_DEFAULT_APEX_PAYLOAD_TYPE=erofs m droid
Change-Id: Iff5f3b77136a303976d1aec992d048240c1adf52