Make BaseDexClassLoaderTest independent of its parent class loader

Update the test to use a well known class loader chain which does not rely
on its running context.

Use the system class loader instead of the actual test class loader as the
parent in the test.

Bug: 38138251
Test: cts-tradefed run cts -m CtsLibcoreTestCases -t
libcore.dalvik.system.BaseDexClassLoaderTest

(cherry picked from commit 3d166bdc)

Change-Id: If0d8b8f87cf936a687affb1429b65236e3a75500