[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.cpp

@@ -77,8 +77,8 @@ void AndroidRendererFrontend::update(std::shared_ptr<UpdateParameters> params) {
     mapRenderer.requestRender();
 }
 
-void AndroidRendererFrontend::onLowMemory() {
-    mapRenderer.actor().invoke(&Renderer::onLowMemory);
+void AndroidRendererFrontend::reduceMemoryUse() {
+    mapRenderer.actor().invoke(&Renderer::reduceMemoryUse);
 }
 
 std::vector<Feature> AndroidRendererFrontend::querySourceFeatures(const std::string& sourceID,

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;

platform/android/src/native_map_view.cpp

@@ -450,7 +450,7 @@ jni::Array<jni::jlong> NativeMapView::addMarkers(jni::JNIEnv& env, jni::Array<jn
 }
 
 void NativeMapView::onLowMemory(JNIEnv&) {
-    rendererFrontend->onLowMemory();
+    rendererFrontend->reduceMemoryUse();
 }
 
 using DebugOptions = mbgl::MapDebugOptions;

platform/darwin/src/MGLRendererFrontend.h

@@ -56,9 +56,9 @@ public:
         return renderer.get();
     }
     
-    void onLowMemory() {
+    void reduceMemoryUse() {
         if (!renderer)  return;
-        renderer->onLowMemory();
+        renderer->reduceMemoryUse();
     }
     
 private:

platform/ios/src/MGLMapView.mm

@@ -716,7 +716,7 @@ public:
 {
     MGLAssertIsMainThread();
 
-    _rendererFrontend->onLowMemory();
+    _rendererFrontend->reduceMemoryUse();
 }
 
 #pragma mark - Layout -
@@ -2315,7 +2315,7 @@ public:
 
 - (void)emptyMemoryCache
 {
-    _rendererFrontend->onLowMemory();
+    _rendererFrontend->reduceMemoryUse();
 }
 
 - (void)setZoomEnabled:(BOOL)zoomEnabled