[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/annotation/render_annotation_source.cpp

@@ -73,8 +73,8 @@ std::vector<Feature> RenderAnnotationSource::querySourceFeatures(const SourceQue
     return {};
 }
 
-void RenderAnnotationSource::onLowMemory() {
-    tilePyramid.onLowMemory();
+void RenderAnnotationSource::reduceMemoryUse() {
+    tilePyramid.reduceMemoryUse();
 }
 
 void RenderAnnotationSource::dumpDebugLogs() const {