Fix variable interpolation to be PHP 7 Compatible

Problem/Motivation

https://www.drupal.org/node/2454439#comment-10160036 identified some code that uses variable referencing that is not compatible with PHP 7. The fix is fairly straight forward.

Reference on the PHP7 Change:
https://wiki.php.net/rfc/uniform_variable_syntax

                        // old meaning            // new meaning
$$foo['bar']['baz']     ${$foo['bar']['baz']}     ($$foo)['bar']['baz']
$foo->$bar['baz']       $foo->{$bar['baz']}       ($foo->$bar)['baz']
$foo->$bar['baz']()     $foo->{$bar['baz']}()     ($foo->$bar)['baz']()
Foo::$bar['baz']()      Foo::{$bar['baz']}()      (Foo::$bar)['baz']()

Proposed resolution

Just split the code up to make it more clear.

Remaining tasks

Review, commit.