[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.

include/mbgl/renderer/renderer.hpp

@@ -48,7 +48,7 @@ public:
     void dumpDebugLogs();
 
     // Memory
-    void onLowMemory();
+    void reduceMemoryUse();
 
 private:
     class Impl;