Add backtrace_string and export to libmemunreachable

Add backtrace_string to convert a malloc_debug backtrace to a string.
Also move the backtrace functions to libc_malloc_debug_backtrace so that
libmemunreachable can reuse them.

Bug: 27208635
(cherry picked from commit 2c759913)

Change-Id: Ia93aaf010fbce1ace058439427066ea2dfd8bd13