Code cleanup: StagedRollbackTest (1/n)

Rename device side methods to testBadApkOnly_PhaseN to show that they
are grouped and coordinated by the host side method testBadApkOnly.

Bug: 142697546
Test: atest StagedRollbackTest
Change-Id: I54f28f545a7bd1fb4859421dbdaf1d95fd7c4a9d