When whitelisting content types to debug print from session we chose
application/json and application/text. application/text is not a real
mime type, text is typically text/plain.
Rather than guess at mime types only print application/json to start
with, but make it easy for additional types to be added later.
Change-Id: Ica5fee076cdab8b1d5167161d28af7313fad9477
Related-Bug: 1616105