Dialer: Add a helpline phone book

* Expose the sensitive numbers to the user so they can easily call one
  of them
* Make sure to leave as few traces of access as possible:
  * Calling a number closes the helpline activity
  * Opening a link closes the helpline activity
  * Before opening a link, a warning about browser history appears

* This makes use of a changed xml-format, which now allows adding
  various things:
<item>
    <categories>violence|human trafficking</categories>
    <name>Some NGO or whatever</name>
    <number>1234</number>
    <anything>anything else</anything>
</item>

Change-Id: I952bf10ee9516b83e595d8a19450696b303393b6