Replace unused dependency on libmemunreachable

libmediautils was accidentally getting backtrace_string through
libmemunreachable.  Remove the dependency, which is causing problems
for making libmemunreachable vendor_available, and use
libc_malloc_debug_backtrace instead.

Bug: 132302484
Test: m libmediautils
Change-Id: Ide83ce43027f28d548a76c607a84bab6604213cf