Fix libmemunreachable vs hwasan conflict.

* Scan memory in a non-sanitized function.
* Don't scan hwasan shadow mappings.
  Shadow is ~33G and never contains any heap pointers. The scan takes
  about a minute on my device.

Bug: 112438058
Test: memunreachable_test with SANITIZE_TARGET=hwaddress
      the test still fails, but now there are no hwasan reports or new
      tombstones, and it completes in less than a minute

Change-Id: Ibe4811c67609ca9fe317edc250993b915570a638