Storage Settings: Allow user to trigger a volume rescan

This change allows to trigger a rescan of a volume with the next restrictions:
- Internal volumes if they are emulates
- Primary volumes
- Non-Removable volumes

Ported from: http://review.cyanogenmod.org/44586

Change-Id: I7e75a324938d8081fd715f702d50f1fdceabd96d
Clean up lock screen interface code

- Remove unnecessary (and wrong) user check for the removal of maximize
  widgets

- Remove unnecessary null checks for the widgets category

- Reorder preference initialization to the same order as they appear on
  screen

- Extract DPM checking into a function for future additions, remove
  unused DPM admin class, centralize LockPatternUtils, clean up comments
  and wrap to 100 lines.

Change-Id: I94159fec7b1604e290829d2ddc2eb07bf5b015e7