Updater: Switch to destination selector

* Let the user decide where to store the file
* That way it's not located in /storage/emulated/0/Android/data/...
  ...org.lineageos.updater/files/LineageOS updates/*.zip
  -> The user knows where the file is stored
  -> We don't have to care about WRITE_EXTERNAL_STORAGE etc
* Remove the cancel button - after closing the file stream we loose
  permission to access it, therefore can't delete it anymore
  -> Let the user handle deletion manually
* Since we don't use WRITE_EXTERNAL_STORAGE anymore, remove it from
  Manifest and also remove PermissionUtils (+calls) - we can now export
  immediately.
  -> This also solves the
  "TODO: start exporting once the permission has been granted"

Change-Id: I50afa403f2803569aa9def807ea20ee72c582284