Fix build part 2. Always create platform_mapping_file.

commit 552fb537 fixed an undefined
module error by removing the module when not defined (on non-treble
devices), but the sepolicy build on non-treble devices was changed
to rely on the split treble files, even though the split is not used.
Change this so that the file is always present, to allow policy
compilation.

Test: policy fully builds.
Change-Id: Ia0934c739336cea54228bbff8d6644aa3ae501e5