Use new RollbackManager API in RollbackPackageHealthObserver.

Use RollbackManager.getAvailableRollbacks instead of
RollbackManager.getAvailableRollback, which will be removed shortly.

Bug: 112431924
Test: atest RollbackTest
Change-Id: Ia6548e4d66da3484cb90ab6f20832356628f1881
1 file changed