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/HeapWalker.h b/libmemunreachable/HeapWalker.h
index 865965a..5c7ec13 100644
--- a/libmemunreachable/HeapWalker.h
+++ b/libmemunreachable/HeapWalker.h
@@ -25,6 +25,8 @@
 #include "ScopedSignalHandler.h"
 #include "Tarjan.h"
 
+namespace android {
+
 // A range [begin, end)
 struct Range {
   uintptr_t begin;
@@ -125,4 +127,6 @@
   }
 }
 
+}  // namespace android
+
 #endif