[e2e][audio] Write canary values to vmo after reading

There is still some interesting glitching in the e2e test.  Write a
canary value to the vmo after reading to see where the glitchy audio
is coming from.

Test: Ran e2e test locally, confirmed it still works.
Change-Id: I83ab3c29c989317d9b98a29996d37fcee44b782a