Fix libnativeloader to correctly link to the platform namespace.

This affected binaries in the Runtime APEX, where the platform namespace is
"platform" rather than "default".

Also extend ANDROID_ADDITIONAL_PUBLIC_LIBRARIES to create links to both to
platform and runtime namespaces, so that it can be used to open up access to
internal libraries in the Runtime APEX as well, which is used by ART gtests
and run tests.

Also update some comments in the ld.config*.txt files to accurately explain
why some namespaces need to be visible, and some other minor changes for
consistency. There are no semantically significant changes in those files.

Test: Flash and boot
Test: Run an ART run test with the internal libarttest.so library
Bug: 130293232
Bug: 121117762
Change-Id: I7ebaf5370dd0f533b1bb5f0e67e7c3c1df48e512