The Android documentation tells me that I can access a string from another package by using the \"package name\", whatever that means:
@[
In short: package name prefix is for shared libraries, not for your apk.
The documentation reads:
To reference a system resource, you would need to include the package name
This corresponds for external shared libraries (and their resources), your application is linked against (e.g. maps). They have their own R class, not merged with that of your application.
These are all standard android packages and shared libraries you mentioned in section of android manifest.
As Nikolay pointed out, all app resources are merged, that is why the majority library projects use prefixes like abs__ for their resource names.
See Library projects doc for additional info.