android: More tests for stale network context cleanup

This is a followup to crrev.com/1014986 to add more tests following the
test structure determined earlier. This does not add a lot of extra code
coverage, rather adding 'constant coverage', which we do not measure.

Expecting this to be the last commit associated with the crbug. Fingers
crossed.

PushChangedFiles times out on "test new tests for flakiness", so:

Validate-Test-Flakiness: skip
Bug: 1331809
Change-Id: Ie641994570aae4e17f465bc60e1e61445802bac2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3714181
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1019114}