refactor($animate): avoid unnecessary lookup

[gkalpak]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Refactoring. (Theoretically a perf improvement, but it is so insignificant, that it's not worth mentioning as such 😛 )

What is the current behavior? (You can also link to an open issue here)
In $animate.enabled(), we look up in disabledElementsLookup even if we never use the value.

What is the new behavior (if this is a feature change)?
In $animate.enabled(), we look up in disabledElementsLookup only if we need the value.

Does this PR introduce a breaking change?
No.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
No 😕

[petebacondarwin]

LGTM