Optimize notification large icon and Android Wear background sizes

Large icon size is set by android.R.dimen.notification_large_icon_height and changed between pre-Android 5.0 and 5.0. To ensure the best large icon, we use the provided dimension - Android by default does not center crop too large of images so the fixed size was giving strange results, particularly on low resolution devices.

While there isn't a set size for BigPictureStyle backgrounds, this background is used on Android Wear devices, which suggest a 400x400px background per http://developer.android.com/training/wearables/notifications/creating.html#AddWearableFeatures - we load a separate background image of this size.
Merge pull request #81 from ianhanniballake/notification_background_size

Optimize notification large icon and Android Wear background sizes