ART: Re-add dlopen

Re-add an oat-file path that uses dlopen to load oat files. This
code-path will be necessary to support libunwind unwinding through
properly CFI-annotated oat files, as libunwind consults the linker
about loaded ELF files.

We avoid the original dlopen issue, namely that the semantics of
dlopen disallow loading the same soname twice, by using an extension
of bionic that supersedes the specified behavior.

Change-Id: I4a7e3cb00d1ea156dad84f76826def2a5967c9ca