transcoding: drop transcoding request for bad actors

Drop requests for a uid if it submits more than X back-to-back requests
that lasted longer than Y seconds.

bug: 177631807
test: new and existing unit tests; CTS MediaTranscodeManagerTest;
Manual testing Yelp app (with shortened thresholds)

Change-Id: I88287be62190ee5bc815dedb2a62c54ed3c41824