Don't fail rollback for missing installer packages.

There is no need to preserve the installer package if there isn't any in
the first place.

Bug: 112431924
Test: adb install --enable-rollback RollbackTestAppAv2.apk
      adb shell pm rollback-app com.android.tests.rollback.testapp.A

Change-Id: I3c640866ca14dd3cdef5e334de7eee2176f632bf