[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.cpp

@@ -94,9 +94,9 @@ void Renderer::dumpDebugLogs() {
     impl->dumDebugLogs();
 }
 
-void Renderer::onLowMemory() {
+void Renderer::reduceMemoryUse() {
     BackendScope guard { impl->backend };
-    impl->onLowMemory();
+    impl->reduceMemoryUse();
 }
 
 } // namespace mbgl