Code cleanup: StagedRollbackTest (4/n)

Add device side methods with names like testFoo_PhaseN to show they are
grouped and coordinated by the same host side method testFoo.

Bug: 142697546
Test: atest StagedRollbackTest
Change-Id: Ic001c94075cbf5fc455188f5ecbaf73d58306e5f
2 files changed