Ensure loaded plugins are not copied.

If a loaded plugin was copied we would try to initialize it a second
time. This would happen if you were running on a libartd.so because
the compiler would not select the move constructor since it wasn't
marked as noexcept.

Bug: 74125483
Test: Run dexmaker tests on a libartd runtime.
Change-Id: If86e4d3ed8fc68fce7295fb3779cb46499f9c72c