Putting libmemunreachable in the global C++ namespace was an oversight,
move it into namespace android.
Test: m -j checkbuild
Change-Id: I0799906f6463178cb04a719bb4054cad33a50dbe
libmemunreachable uses an imprecise mark and sweep pass over all memory
allocated by jemalloc in order to find unreachable allocations.
Bug: 27208635
Change-Id: Ia70bbf31f5b40ff71dab28cfd6cd06c5ef01a2d4
(cherry picked from commit bcb4ed3eaa)