Make sense out of scan & parse flags

Parse flags are often used confusingly to control policy during
APK parsing. Additionally, some flags [such as PARSE_IS_PRIVILEGED
or PARSE_IS_SYSTEM] aren't always determined based upon APK location
[they change based upon which apps are installed]. This means we
have to perform some gymnastics to set them correctly _before_
parsing a package.

Instead, these should be scan flags that can be determined at a
later point during package parsing. Then, based upon the scan flag
we apply policy on the package object to create the resulting
data structures.

Bug: 63539144
Bug: 69663125
Test: Manual; it builds, runs and applications can be installed / removed
Test: bit FrameworksServicesTests:com.android.server.pm.
Change-Id: I53fb18e58544e0a39f35eb6fb53c950e5c696aee