MountService: allow overriding default storage list with ro.storage_list.override

Useful for devices with different storage layout variants, e.g. with
either emulated or non-emulated sdcard0.

Change-Id: Ia44ef6243563a69cb8d38a7b87bd81cd37f1df63
services: AssetAtlasService: prevent IndexOutOfBoundsException

Change-Id: I319903e8d763b9f48dc675676f27f3fadc4f3de5