Make RollbackPackageHealthObserver observe apk-in-apex

Due to b/147666157, this CL will only work as long as an apk is not
embedded in two different apex.

Bug: 142712057
Test: StagedRollbackTest#testRollbackApexWithApkCrashing
Change-Id: I60f7f70b627fd380094a11373259debddd5dc532