Increase sleep timeout of testNetworkFailedRollback_Phase3 (4/n)

My local tests show that sometimes testNetworkFailedRollback_Phase3
fails to reboot within 120s. Since it takes 120s to fail the health
check and trigger rollback, there is not much time left to finish
shutdown and begin reboot before finishing sleeping.

Let's generously give it 120 more seconds so shutdown and reboot have
enough time to take place.

Bug: 143999213
Test: atest StagedRollbackTest
Change-Id: Icf3a5583a906da312189a6d027a5c269e2fd8d46