Clean up generatePackageInfoFromApex() API

This changes the API for generatePackageInfoFromApex in PackageParser
in response to review suggestions made in ag/6701090.

The method is renamed to generatePackageInfo with a PackageParser.Package
parameter (as well as an ApexInfo and flags). The implementation is combined
with the main generatePackageInfo method. Callers are changed to perform
the parsing themselves, including the collection of certificates if
necessary.

Note: the method signature and implementation had already changed since
ag/6701090, so the suggestions from that CL may not apply directly.

Bug: 129261524
Test: atest PackageParserTest
Test: atest CtsStagedInstallHostTestCases
Change-Id: I495539679812110f89d5e3d93f2622ba2dab36c0