Avoid re-mapping dex file that's in local memory.

If the Dex file we're trying to examine is already within the unwinder's
address space, we don't need to load it from disk or copy it across
processes.

This avoids using up virtual address space to map in dex files, and
also should be a bit faster to read since it won't go out to the file.

Patch by Chris Sarbora

Test: Ran new unit tests.
Test: Ran 137-cfi art test.
Change-Id: I949457856f051cca11b9020e9da3a41bbf6e5c8e