Add cert digest separator to pm app link output

For easy copy-paste if someone is configuring their server to host an
assetlinks.json.

Bug: 184954099

Test: manual, pm get-app-links on device

Merged-In: I5ed3505d72b5522202743a49559fbf1230b5767e
Change-Id: I5ed3505d72b5522202743a49559fbf1230b5767e