Code cleanup: StagedRollbackTest (2/n)

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

Also put related methods together for readability.

Bug: 142697546
Test: atest StagedRollbackTest
Change-Id: I3186d27c4d3c5176f72d72f1682ed46ae0cb62ea