[core][android] Provide API to control eviction of cached images

[alexshalamov]

This PR introduces following changes:

• Defines constant for ImageManager's cache size DEFAULT_ON_DEMAND_IMAGES_CACHE_SIZE that is equal to ~100 8kB sprites.
• Adds bool onCanRemoveUnusedStyleImage(const std::string&) method to MapObserver interface that is invoked in two cases: OOM and when cumulative size of unused images in the cache goes over DEFAULT_ON_DEMAND_IMAGES_CACHE_SIZE limit.
• Android implementation and unit tests

By default, map will remove *unused images if application returns true from onCanRemoveUnusedStyleImage method.

*Unused images: are images whose requesting tile is destroyed. Map will notify about unused images only if image was provided through onStyleImageMissing(id) => addImage(id).

// Android example
mapView.addOnCanRemoveUnusedStyleImageListener(id -> {
  return id != "keep_icon";
});

Fixes: #14547

---

TODO:

• Darwin platform bindings

[alexshalamov]

/cc @julianrex Would someone from your team have time to implement darwin platform bindings?

[pozdnyakov]

why the above code not needed?

[pozdnyakov]

I see it moved to the checkMissingAndNotify, could you give some details on this?

[julianrex]

@alexshalamov yes, we can look at the Darwin side :)

[julianrex]

@alexshalamov @tobrun a couple of questions:

1. What's the policy for evicting images? Just all those that haven't been used recently? Is it an LRU policy?

2. I wonder if this should ask platform code if it should evict images before doing so? An application may have good reason to keep an image around - is there a way to do this already?

[alexshalamov]

@julianrex

1. What's the policy for evicting images? Just all those that haven't been used recently?

Whenever cumulative size of unused images that were added via onStyleImageMissing API goes over the limit, platform will notify MapObserver to remove unused images. By default, core will remove images from the style.

2. I wonder if this should ask platform code if it should evict images before doing so? An application may have good reason to keep an image around - is there a way to do this already?

When core notifies SDK / application code and there is developer provided handler for the onRemoveUnusedStyleImages notification, core will not remove anything and delegate it completely to the application.

[julianrex]

@alexshalamov can this land without the iOS parts? Or would you like a PR to this branch?

[alexshalamov]

@julianrex not necessary to this branch, could be separate PR, I just want to make sure that the new interface is implementable for darwin platform.

[fabian-guerra]

On the Darwin side we have all what we need to build the bindings. I just have a question regarding the implementation.

@alexshalamov @julianrex we could pr the iOS bindings here or in a new pr, your call.

[chloekraw]

@alexshalamov quick point of clarification - when you say

Whenever cumulative size of unused images that were added via onStyleImageMissing API goes over the limit, platform will notify MapObserver to remove unused images.

that means all unused images are removed? so this has no direct relationship to least recently used, correct?

[alexshalamov]

@chloekraw

that means all unused images are removed? so this has no direct relationship to least recently used, correct?

All unused images that were added as a result of onStyleImageMissing notification. As for recently used, I don't think we have such concept at the moment.

[alexshalamov]

@tobrun @fabian-guerra are you fine with the new observer method? I would like to merge PR this week.