[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.

src/mbgl/renderer/render_source.hpp

@@ -70,7 +70,7 @@ public:
     virtual std::vector<Feature>
     querySourceFeatures(const SourceQueryOptions&) const = 0;
 
-    virtual void onLowMemory() = 0;
+    virtual void reduceMemoryUse() = 0;
 
     virtual void dumpDebugLogs() const = 0;