Add network health check watchdog triggered rollback tests

This cl improves code coverage for the PackageWatchdog explicit health
checks.

Updating the NetworkStack triggers an explicit health check with
a time deadline. At the deadline, if network is unavailable on the device,
the update is rolled back, if network is available, nothing happens.

Bug: 132640467
Test: atest StagedRollbackTest

Change-Id: I16a40244de9345930a1c981188aa33546fe872fc