Rollback info API change. Added impact level to RollbackInfo.

Rollbacks are currently stored only for some mainline packages, but
rollbacks will be added for phonesky and GMS core in V. These new
rollbacks should not be treated similar to mainline. We would want to
perform GMS core rollback only when all other rescue party mitigations
are already tried. And Phonesky should not be automatically rolled back.

We introduce a new field rollbackImpactLevel to RollbackInfo, which is
used to determine the order in which these rollbacks should be performed
relative to other mitigations in PackageWatchdog.

rollbackImpactLevel is exposed through PackageInstaller.SessionParams.

Bug: 291135724
Test: CTS test. Unit tests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:a3ae2464aac43d8bbc2129bdea7733d09b90e2da)
Merged-In: I723405eb62577202893ff208af2ab01b212b112a
Change-Id: I723405eb62577202893ff208af2ab01b212b112a