Added high impact rollback in RollbackPackageHealthObserver

Added high impact rollback as a last resort after low impact rollbacks
are tried. Also removed reference to automatic rollback deny list, since
we will be using impact level instead of the deny list going forward.

Bug: 291137901
Test: Unit tests
Change-Id: I4d75dd513b3f43c179baf93ece832f9007173ff4
7 files changed