Add `ld.config.txt` file to Android Runtime APEX packages.

This file is required when the Runtime APEX directories are part of
Bionic search paths.

The `ld.config.txt` file added by this change is currently empty.
Follow-up commits will add actual content to it.

Test: Device boot test with Android Runtime APEX.
Test: Device boot test without Android Runtime APEX.
Test: cts-tradefed run cts --m vm-tests-tf
Bug: 113373927
Bug: 119949783
Change-Id: I3df75b1460a43b410663ad816b5e01b9846b3fcb