Bug 1079322 - Extract properties using libc functions. r=gwagner

The LogShake features allows one to shake its device to be able to dump
a set of useful logging informations. This includes several log files,
and also the Android properties. In the past, we relied on the
/dev/__properties__ file to extract their content by parsing its value.
This is duplicate work from the bionic libc and libcutils library.
Worst, the format used to store the values in this file has been changed
between JellyBean and Kitkat, so our parser was not able to dump the
values: that explains bug 1079322. To fix this we make use of some of
the underlying libc-defined functions used to iterate and get properties
values:
 - __system_property_find_nth() to retrieve one arbitrary property by
   its number (starting from 0), and this returns a struct containing
   all the informations
 - __system_property_read() to read the values contained in the struct
   that was previously retrieved
Bug 1095617  - Don't break when loggedInUser == 'null'. r=ferjm