[iOS] Fix the problem that background color should use [UIColor clear…

[wqyfavor]

…Color] instance when transparent.

Brief Description of the PR

Checklist

• Demo:
• Documentation:

[weex-bot]

	Warnings
⚠️	No Changelog changes! - Can you add a Changelog? To do so,append your changes to the changelog.md
⚠️	Current pr not bind the milestone
⚠️	If your PR is about fixing a bug excluding crash the code,you should add the demo link in the PR description. Demo link: http://dotwe.org/vue
⚠️	If you update the code, maybe you should update the documentation and add the documentation link in the PR description. here is the guide about how to contribute documentation: https://github.com/apache/incubator-weex/blob/master/CONTRIBUTING.md#contribute-code-or-document

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[jianhan-he]

LGTM