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, rollback expiration didn't happen as expected
and caused the test to fail.

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

Note this is the same fix as ag/10467568.

Bug: 151890602
Test: atest RollbackTest
Change-Id: Ie884e4807999d105f6224c46a39434e1f437ed1b