Add support for MTP/PTP (2/2)

* MTP/PTP allows you transfering your media files without interrupting
  internal sdcard functionality. You can use your android File Manager
  and transfer files to your computer at the same time

* USBMS allows you transfering any files, it unmounts /sdcard
  and breaks running android. You can't take picture while transfering
  files to your computer

* USBMS enabled by default. To change the default behaviour,
  Settings - Storage - MENU - USB computer connection

Change-Id: Ic23051826a402f53fd1e5d7ac2f00f906972d02c