Use system properties to log rollback events

Sometimes logcat messages are missing and causing rollback tests
to fail which check logcat messages to confirm some particular
rollback events have happened.

Let's use system properties as a reliable way to log rollback
events.

Bug: 171853854
Test: atest StagedRollbackTest NetworkStagedRollbackTest
Change-Id: Ia74da915811978008eed501049f5f44029e5c81a