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Fix StackOverflowError when merging ParseObject from JSON (#925)

* Add test for StackOverflowError during mergeFromServer

This test shows the #896 bug.

* Make a synchronized copy of availableKeys

`synchronizedSet(s)` ends up making a new wrapper for the supplied set, which
means on subsequent calls, it becomes a SynchronizedSet, wrapped in a
SynchronizedSet, wrapped ... etc.

When using the LocalDataStore, the ParseObject.State is reused for every
matching ClassName+ObjectId pair, so every time a ParseObject is parsed from
JSON data, the existing object is "refreshed" by making a copy of its State,
and then merging with the new data. Every call to State.newBuilder() is
therefore adding a new layer of SynchronizedSet to the availableKeys Set.

Eventually that nested hierarchy of sets becomes so large that it causes a
StackOverflowError for any operation on the collection.

By making a copy of the set before wrapping it in synchronizedSet(), that
nested hierarchy becomes severed, and we end up with just one level of
wrappers.

* Accept the updated Android Build-Tools 27 license

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