Fix a flaky test

See b/152840581#comment2 for the analysis.

1. call #waitForDeviceNotAvailable to wait for reboot reliably.
2. reduce watchdog timeout to 120s which works well for me and speeds up
   the test.

Bug: 152840581
Test: atest StagedRollbackTest
Merged-In: Ibbc9998c74681b1b8fc29b90ea66406f39ae7443
Change-Id: Ic791fbc00c231a5f24751b36e99515cfad13b7e7