Merge pull request #18521 from etiennesegonzac/bug-997924-callscreen-fix-visibility

Bug 997924 - Make sure the preloaded callscreen doesn't keep an audio channel open
Conflicts:

	apps/callscreen/js/index.js
	apps/system/js/dialer_agent.js
Merge branch 'v1.3t' of https://github.com/leonazhao/gaia into v1.3t_0423_2

Conflicts:
	apps/system/js/dialer_agent.js