Add /system/lib to the permitted paths for ns of APEXes

With b/144533348, there will be symlinks from libs in APEXes to the libs
in the system partition. This is to reduce the size of APEXes when the
APEX is bundled with the platform.

Adding the /system/lib to the permitted paths so that the realpaths of
the symlinks are allowed for the namespaces.

Note that this however does not open all libs in the system partition to
the APEX namespaces, because searching of the libs are NOT done in
/system/lib, but in /apex/<module>/lib directory.

Bug: 144533348
Test: m
Change-Id: I4bb1d6cb5dbf647ae669f49aa5ccb178f5488b5e