[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/sources/render_vector_source.cpp

@@ -88,8 +88,8 @@ std::vector<Feature> RenderVectorSource::querySourceFeatures(const SourceQueryOp
     return tilePyramid.querySourceFeatures(options);
 }
 
-void RenderVectorSource::onLowMemory() {
-    tilePyramid.onLowMemory();
+void RenderVectorSource::reduceMemoryUse() {
+    tilePyramid.reduceMemoryUse();
 }
 
 void RenderVectorSource::dumpDebugLogs() const {