fix(ngAnimateSwap): make it compatible with `ngIf` on the same element

Previously, both `ngAnimateSwap` and `ngIf` had a priority of 600, which
meant that (while both are [terminal directives][1]) they were executed
on top of each other (essentially messing each other's comment node).

This commit fixes it, by giving `ngAnimateSwap` a priority of 550, which
is lower than `ngIf` but still higher than other directives.

For reference, here is a list of built-in directive per priority:

```
-400: ngInclude, ngView
  -1: ngRef
   1: ngMessage, ngMessageDefault, ngMessageExp, ngModel, select
  10: ngModelOptions
  99: ngHref, ngSrc, ngSrcset
 100: attr interpolation, ngChecked, ngDisabled, ngList, ngMax,
      ngMaxlength, ngMin, ngMinlength, ngModel (aria), ngMultiple,
      ngOpen, ngPattern, ngProp*, ngReadonly, ngRequired, ngSelected,
      ngStep, ngValue, option
 400: ngInclude, ngView
 450: ngInit
 500: ngController
 600: ngAnimateSwap, ngIf
1000: ngNonBindable, ngRepeat
1200: ngSwitchDefault, ngSwitchWhen
```

[1]: https://docs.angularjs.org/api/ng/service/$compile#-terminal-

Fixes #16616

Closes #16729