Revert "Remove execmod support for newer API versions"

We need more time to investigate the effect that this change will
have on DRM solutions. Until the investigation is done, revert.

This reverts commit 38d3eca0.

Bug: 30146890
Bug: 20013628
Bug: 35323421
Change-Id: I5ad69ef5ee12081ce7fc0a8440712f7f8f77cf16
Test: policy compiles.