Switch libbacktrace new unwinder.

Update the backtrace leak tests to share a map since this is the most
common way it will be used, and it runs much faster.

Remove the CreateNew functions, and references to them.

Remove benchmarks of CreateNew functions.

Test: Builds, ran new unit tests, ran art tests.

Change-Id: I4a25a412f1a74c6d43aebbebbf32ead20ead8f94