Initial implementation of StorageManager.getVolumesList().

This change makes StorageManager.getVolumesList(),
StorageManager.getPrimaryVolume(),  and StorageVolume public and adds a
buildAccessIntent() in the latter to automatically generate the
ACTION_OPEN_EXTERNAL_DIRECTORY intent, but it doesn't change the
ACTION_OPEN_EXTERNAL_DIRECTORY implementation yet (i.e., it still takes an URI with the physical path of the directory, instead of a StorageVolume and
a directorny name).

BUG: 26742218

Change-Id: I36c59c42b6579e125ec7f03c3af141260875a491