[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/renderer_impl.cpp

@@ -722,10 +722,10 @@ std::vector<Feature> Renderer::Impl::querySourceFeatures(const std::string& sour
     return source->querySourceFeatures(options);
 }
 
-void Renderer::Impl::onLowMemory() {
+void Renderer::Impl::reduceMemoryUse() {
     assert(BackendScope::exists());
     for (const auto& entry : renderSources) {
-        entry.second->onLowMemory();
+        entry.second->reduceMemoryUse();
     }
     backend.getContext().performCleanup();
     observer->onInvalidate();