Fix up PackageSetting pkg and path documentation

Renames baseCodePath -> baseApkPath and codePath -> path to better
reflect their actual meanings. Their respective JavaDoc has also been
consolidated and updated.

Also explains why the pkg field can be null, as this has been the cause
of a significant number of bugs. Also hides it behind a getter to
preempt any mutation issues. Doesn't refactor existing usages. This can
be done inline with other changes that touch that code.

Bug: 164218844
Bug: 164488334

Test: atest com.android.server.pm

Change-Id: Ia254e7cebb0ab86165e5ec42e8f8bfcf8673884b