Make dlext.ns_shared be independent of default namespace config

dlext.ns_shared creates a child namespace, which is isolated & shared,
from the default namespace. It then checks whether a lib that was loaded
in the parent(=default) namespace *after* the child namespace is created
is not accessible to the child namespace. Because this test is using the
default namespace as a parent namespace, the test result is dependent on
the specific configuration of the default namespace, which isn't
desirable.

Specifically, since ANDROID_NAMESPACE_TYPE_SHARED now copies search
paths and permitted paths of the parent namespace to the child namespace,
a lib that is accessible to the former is also accessible to the latter
even if the lib hasn't been shared when creating the latter. So, the
result of the test varies depending on whether the default namespace has
permitted paths or not. (if it is configured as an isolated namespace
and has permitted paths, then the test fails).

In order to make this test be independent from how the default namespace
is configured, a parent namespace is explicitly created and the child
namespaces are created from the parent namespace, not from the default
namespace.

Bug: 65189955
Test: bionic-unit-tests --gtest_filter=dlext.ns_shared successful
Test: the same test successful when the default namespace is configured
as follows
namespace.default.isolated = true
namespace.default.permitted_paths = ...:/data

Change-Id: Ifa33c6c42f8c327b2b1c380c7abba8e8f1afee82