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.

Ayysir
===
   -- Implemented CM's way in more category

Change-Id: I983eaf8f15a152ff18bb16cfa857ca911b2c7e19