General optimization.
It's more a wallpaper timer, a folder containing images is chosen and used as wallpaper by alphabetical order each predefined time.
This is NOT a live wallpaper neither a service, so don't stay in memory, the app only changes the wallpaper and deactivate itself up to the next cycle.
The ideal image should have the height of the device and twice the width, if not the image will be re-sized but will require more CPU power and time.
There are three permissions, one to change the wallpaper (android.permission.SET_WALLPAPER), other to activate on boot only the minimum time required to schedule the next wallpaper change event (android.permission.RECEIVE_BOOT_COMPLETED) and read SD-card is necessary for KitKat (android.permission.READ_EXTERNAL_STORAGE).
