feat(cluster): Support updating existing items

[barbeau]

As discussed in #90, when an item instance is removed, updated, and re-added the ClusterManager, the updated item state wouldn't be reflected on the map.

The reason for this is that DefaultClusterRenderer.CreateMarkerTask.perform() was using a marker from its own mMarkerCache associated with that item to display that item on the map (even if ClusterManager.clearItems() was previously called). This resulted in the updated item contents not being reflected on the map, because the cached marker was not updated.

This PR does the following:

• Add new protected methods in DefaultClusterRenderer - onClusterItemUpdated() and onClusterUpdated() - which are called when an existing cached marker is found for an item/cluster when that item/cluster is being added back into the ClusterManager. The default implementation of these methods update the marker contents on the map, but then can also be overridden by applications for custom behavior (similar to onBeforeClusterItemRendered() and onBeforeClusterRendered()). Following this change, you can now do ClusterManager.clearItems() and re-add the same list instance (or remove() and then add() the same item) and then call cluster() and the map markers will be updated with the new item model contents.
• Add a new ClusterManager.updateItem() helper method for updating the ClusterManager and algorithms with an existing item. The current implementation in NonHierarchicalDistanceBasedAlgorithm is a wrapper for remove() and then add().
• Add more descriptive Javadocs to ClusterManager, DefaultClusterRenderer, and NonHierarchicalDistanceBasedAlgorithm to describe expected behavior, especially that cluster() should be invoked after any changes to ClusterManager to see changes on the map.

One new TODO I added that could potentially be addressed - it seems like the code to set the marker text when creating the item marker should be refactored into onBeforeClusterItemRendered() to match the implementation of clusters (onBeforeClusterRendered()), and to allow applications extending DefaultClusterRenderer to inherit this default behavior via a call to super.onBeforeClusterItemRendered(). However, moving this code would change the behavior of the library for apps already extending DefaultClusterRenderer that aren't calling super.onBeforeClusterItemRendered() - the marker title and snippet would no longer be updated. Thoughts on this are welcome!

Fixes #90

[arriolac]

See comment on the implementation of update and returning a bool.

[codecov]

Codecov Report

Merging #627 into master will increase coverage by 0.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
+ Coverage   18.44%   19.00%   +0.55%     
==========================================
  Files          71       71              
  Lines        4011     4056      +45     
  Branches      591      610      +19     
==========================================
+ Hits          740      771      +31     
- Misses       3237     3251      +14     
  Partials       34       34              

Impacted Files	Coverage Δ
...ng/algo/NonHierarchicalDistanceBasedAlgorithm.java	58.18% <0.00%> (+23.80%)	⬆️

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[barbeau]

Merging master branch to see if that enables CodeCov on this PR...

[arriolac]

LGTM. Before we merge, please follow-up on our discussion https://github.com/googlemaps/android-maps-utils/pull/627/files#r382051446 If we refactor, let's file an issue and tackle in a separate PR, otherwise, we can remove the TODO