Abandon sessions before/after running tests

See b/160281947#comment2 for the analysis.

Let's clean up staged sessions to improve test isolation.

Bug: 160281947
Test: atest StagedRollbackTest
Change-Id: I3ad6317eaee3ffb4c6351c630460dd8f1d27e644
Merged-In: I3ad6317eaee3ffb4c6351c630460dd8f1d27e644
(cherry picked from commit be2c3fc558e76187200bfc264c765a33b9564e17)
3 files changed