Override phenotype flags during tests

https://g3doc.corp.google.com/experiments/g3doc/mobile/phenotype/android/testing.md?cl=head#hermetic

Phenotype talks to a server to override the device config changes made
by the tests. Therefore, watchdog timeout didn't happen as expected and
caused tests to fail.

We need to override the flags on our own during tests so the
device config values ('watchdog_request_timeout_millis' in this case)
won't be changed in an unexpected way.

See b/148042585#comment21 for more details.

Bug: 148042585
Test: atest NetworkStagedRollbackTest
Change-Id: I4d8944f7fef93445de8f971c6a96fef7c71872db