Optimize CompositeBackgroundDrawable

[jorge-cab]

Summary: Its not optimal to reconstruct CompositeBackgroundDrawable everytime we add a layer to it.

With this change I'm adding an optimization to modify the underlying LayerDrawable in place instead of reconstructing everything. LayerDrawable has a pretty constrained API and some weirdish behaviors.

• addLayer - This API doesnt set the callback for the recently added layer so after adding the layer we also need to manually set the callback
• Mutating LayerDrawable in-place is not straightforward, I had to add a function that will figure out where each layer should be inserted
• LayerDrawable doesn't allow deleting an element which means that for this case in particular we do need to re-create CompositeBackgroundDrawable
• Newer Android versions allow null on LayerDrawable layers, but older versions do not, this implementation is mostly done this way to accommodate older versions. But also, even though newer versions can have null set on a layer LayerDrawable still doesn't handle it well and we get some bugs when removing and inserting a layer

This is all feature flagged. since it will only be enabled with the new drawables

Changelog: [Internal]

Differential Revision: D65907786

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This pull request was exported from Phabricator. Differential Revision: D65907786

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D65907786

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D65907786