Move libmemunreachable into namespace android

Putting libmemunreachable in the global C++ namespace was an oversight,
move it into namespace android.

Test: m -j checkbuild
Change-Id: I0799906f6463178cb04a719bb4054cad33a50dbe
diff --git a/HeapWalker.cpp b/HeapWalker.cpp
index df16f40..2403ad0 100644
--- a/HeapWalker.cpp
+++ b/HeapWalker.cpp
@@ -28,6 +28,8 @@
 #include "ScopedSignalHandler.h"
 #include "log.h"
 
+namespace android {
+
 bool HeapWalker::Allocation(uintptr_t begin, uintptr_t end) {
   if (end == begin) {
     end = begin + 1;
@@ -173,3 +175,5 @@
 }
 
 ScopedSignalHandler::SignalFn ScopedSignalHandler::handler_;
+
+}  // namespace android