Uninstall testapps before running SecondaryUserRollbackTest

Add target_preparer for uninstalling TestApp.A and TestApp.B before
running SecondaryUserRollbackTest.
Also, fix the tearDown command after each case to uninstall the right
apps.

Test: atest SecondaryUserRollbackTest ; atest SecondaryUserRollbackTest
Bug: 136538996
Change-Id: I3d5decae03bf9d2f9d78a534399214da05a6d372