GpsInfo: added advanced setting to display HDOP instead of FIX

We depend on HDOP as measure of GPS reliability. GPS will report 'GPS 3D Lock' with as few as 4 satellites and an HDOP of 10+. We don't depend on GPS if HDOP values are > 2 where we fly.

Added an advanced option to replace '3D Fix/No Fix' w/ numeric HDOP value in satellite info panel.

![screenshot_2014-07-08-10-21-15](https://cloud.githubusercontent.com/assets/4014433/3511025/400664f8-06ac-11e4-9302-024fd446e8fe.png)


Move to res/values/preferences_keys.xml