[core] Rename "onLowMemory" to "reduceMemoryUse".

Android still calls "reduceMemoryUse" only while handling a low memory event.
iOS, on the other hand, calls "reduceMemoryUse" every time it enters the background.

platform/android/src/android_renderer_frontend.hpp

@@ -39,7 +39,7 @@ public:
     AnnotationIDs queryShapeAnnotations(const ScreenBox& box) const;
 
     // Memory
-    void onLowMemory();
+    void reduceMemoryUse();
 
 private:
     MapRenderer& mapRenderer;