Fix flaky DLP inspect tests

[shubha-rajan]

Increased the timeout period for the sample from 10 to 15 min, to resolve #2302 , #2303 , and #2317. Also refactored the Pub/Sub code in the samples to be more consistent with other samples.

[lesv]

It looks Ok, and compiles, but I'm confused as I don't see the a reference to handleMessage (only the removal of it), nor do I see the code for messageHandler, though it's being set where handleMessage used to be.

I wonder if there shouldn't be some additional comments, but if you think about my comments and are still ok, feel free to merge.

[shubha-rajan]

The method definition for handleMessage was moved to the bottom of the classes, and messageHandler calls it within a lambda (on line 130 of InspectBigQueryTable.java, 131 of InspectDatastoreEntity.java and 117 of InspectGcsFile.java). I agree that the samples might need some additional comments, but I'm copying the pattern already in the Risk Analysis samples, so might as well update them all at once in another PR. I'll merge this one for now.