[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/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;