Document dumpsys meminfo --unreachable, and how to enable
malloc_debug backtraces on a single app.
Test: none
Change-Id: I649afdfff1c7438f74950395639b06c39cae3ca0
libmemunreachable uses an imprecise mark and sweep pass over all memory
allocated by jemalloc in order to find unreachable allocations.
Change-Id: Ia70bbf31f5b40ff71dab28cfd6cd06c5ef01a2d4