Move APEX symlink creation to alternative module

Move APEX symlink creation to alternative module, one that
is targeted at /system not /. Also added comments to reflect
the tenuous connection between the module chosen and the
symlink creation.

Tested with:
rm -rf out/target/product/taimen/system \
  && make droid \
  && ls -l out/target/product/taimen/system/usr

Test: See above
Bug: 122985829
Bug: 123333111
Change-Id: I841dd42827ac2e082505ebf039f40fd394514e54