Use RollbackManager.getAvailableRollbacks in RollbackTest

Instead of getAvailableRollback and getPackagesWithAvailableRollbacks,
which are soon to be removed.

Bug: 112431924
Test: atest RollbackTest
Change-Id: Ia69342a41bebb91a9fa8044ef8d8b495aba7187f
1 file changed