Only look for profiles when we have a profile dir

Right now, it's looking for *.prof files at the top of the source
directory.

For devices that don't specify the profile dir (everything on AOSP,
etc), this saves a couple percent of the globs that we check during
every build (and would re-execute if you add or remove files from the
top directory)

Bug: 159845846
Test: treehugger
Change-Id: I10d43422a2b5ae25e6557d435ecc89fe43536dd7