[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;