uibModal with backdrop:false moves the backdrop up the stack if there is a modal with backdrop already open

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Bug description:

If a modal with a backdrop is already open on the page and an additional modal with backdrop: false is opened, the backdrop is moved up the stack and placed under the new modal. Ideally, opening a modal with backdrop: false would not affect the z-index of an existing backdrop.

Link to minimally-working plunker that reproduces the issue:

https://plnkr.co/edit/vAtlcxQULYVqXEhyzr9F?p=preview

Steps to reproduce the issue:

1) Open a uibModal with a backdrop 2) Open an additional uibModal without a backdrop 3) Observe the backdrop moves up the stack and behind the new modal

Version of Angular, UIBS, and Bootstrap

Angular: 1.6.1

UIBS: 2.5.0

Bootstrap: 3.3.7

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Removing the following block from modal.js backdropIndex() will fix this issue.

        // If any backdrop exist, ensure that it's index is always
        // right below the top modal
        if (topBackdropIndex > -1 && topBackdropIndex < topModalIndex) {
          topBackdropIndex = topModalIndex;
        }