Settings: add device tree settings hook

Allows devices to hook "advanced settings" into the settings list
by providing an apk that is providing the activity 
org.omnirom.device.DeviceSettings

The activity must be labeled "Advanced settings" to get a unified
look and feel.

PS4:
- This is hidden by default, to show, add
  <bool name="has_advanced_settings">true</bool>
  to overlay/packages/apps/Settings/res/values/bools.xml

PS5/6: fix build (accidentally added <header>)

PS6:
- Better detection if supported (don't need to add bool)
--> Automatically enabled for all supported devices
--> Hidden when app is uninstalled
- Title loaded from package -> no localisation needed

PS7:
- re-arrange advanced settings as first entry in DEVICE Menu

PS8:
- fix major derp :)

PS9:
- Move advanced settings hook to botton of DEVICE menu

Change-Id: Iefbcb0b626ee8df62115d1baa4f0ffadbf9dba36