Fix `m product-graph`

`m product-graph` runs a "non-full" build, which skips
some steps to improve performance. One of the steps
it skips is making a .installable_files file. Soong
reads this file and removes any files that were removed
from the list in this file. It errored out if the file
didn't exist. Make soong less strict about the presence
of this file, because if it doesn't exist, there shouldn't
be any installed files to remove either.

Fixes: 168105598
Test: rm -rf out/, m product-graph
Change-Id: I366f7b09d87911f9660d4e08c2d2f097cc04800f