Add file exists logs to testDownloadFile

Some test runs are failing with "FileNotFoundException: open failed:
ENOENT" on openFileDescriptor in DownloadService. As the root cause is
not clear, add some logs to try to narrow down whether the file really
does not exist, and where it disappears when it does not exist.

Bug: 317602748
Test: atest
Change-Id: Ib67fb4a56283e7131da0b33b5cda390aa5cd668e
1 file changed