Revert "Revert "Allow dlopen("/system/lib/<soname>") for pre-Q""

This reverts commit 1906e776
and fixes dlopen(nullptr)

It's enabled for the following .so libraries
libicuuc.so
libicui18n.so

This CL does not move the .so files into a APEX.
Thus, no visible effect before moving .so files.

Bug: 121248172
Test: dlfcn#dlopen_from_nullptr
Test: Perform the following test after moving libicuuc to /apex
      dlopen("/system/lib64/libicuuc.so") for targetSdkVersion < Q
Change-Id: Ia310ea566485fe00ca28e2203f4bffbd92243638