Re-add the block of text explaining the feature.

When the automatic storage manager  is off (or has never been run), we
display a block of text that explains what it does. In order to make
this testable, I've implemented the feature in a PreferenceController
and upgraded the existing AutomaticStorageManagerSettings class to be
closer to the newer Settings IA.

Change-Id: I3f7d20347a6d5a7bae8bffcd2014c3fdcd315b90
Fixes: 63082545
Test: Settings Robotest