Attach logging package to staged rollback IDs

To ensure consistency in pre-reboot and post-reboot metrics, associate
the logging package name of a rolled back train with the staged rollback
id. This uses the assumption that there is at most one logging parent
(train version package) associated with the rollback. This means that
there will be only one ROLLBACK_SUCCESS or ROLLBACK_FAILURE log for each
event.

This change changes the format of the last-staged-rollback-ids file.
The file now has one line for each rollback id, rather than one
line containing all ids.

Test: Manual test of logging flow for a train that is configured
     correctly, and configured incorrectly
Test: atest RollbackUnitTest
Bug: 154911735
Change-Id: Ie942fdb9313137c2dc6e696840262dd457558c6d
Merged-In: Ie942fdb9313137c2dc6e696840262dd457558c6d