Clean up rollbacks after each test (2/n)

testNetworkPassedDoesNotRollback leaves a rollback for the network stack
and we need to clean up all rollbacks before running
testNativeWatchdogTriggersRollback otherwise the test will fail on
devices which doesn't support checkpoint.

See b/146991129 comment#2 for the detail.

Bug: 147259494
Test: atest StagedRollbackTest
Change-Id: Ib9c5e3f56bf31bfbf6f5a415879f9eb1b10d270e