Change the approach to decide whether to upload metrics.

1. Don't upload if the uploader binary is not present.

Bug: b/193703183
Test: Run the following tests locally:
1. run "go test ." in build/soong/ui/build
2. create vendor/google/misc/metrics_uploader_prebuilt/metrics_uploader.sh
and run m nothing to ensure metrics_uploader.sh was called
3. remove the file created in step 2 then run m nothing and ensure it
doesn't attempt uploading

Change-Id: I081a5510b3f30480720c3e7dd235623c805fa7a4