also verify existence of a package

During an OTA, if a system app is replaced, keeping the same
directory but changing the package name, it's possible to have
a phantom disabled system package setting. In this scenario,
check that the disabled system package is actually backed by
a package.

Change-Id: Ifdaa3b3202256f3b039b0281ba6ab9d669d4b359
Fixes: 65367640
Test: Manual
Test: Create /system/app/Foo/Foo.apk
Test: upgrade app [adb install -r com.foo]
Test: Replace /system/app/Foo/Foo.apk w/ a new package name