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/libmemunreachable/Allocator.h b/libmemunreachable/Allocator.h
index 67a068f..837a12b 100644
--- a/libmemunreachable/Allocator.h
+++ b/libmemunreachable/Allocator.h
@@ -27,6 +27,9 @@
 #include <unordered_map>
 #include <unordered_set>
 #include <vector>
+
+namespace android {
+
 extern std::atomic<int> heap_count;
 
 class HeapImpl;
@@ -210,4 +213,6 @@
 using string = std::basic_string<char, std::char_traits<char>, Allocator<char>>;
 }
 
+}  // namespace android
+
 #endif