Use DimmableIconPreference for Add user action

Moved DimmableIconPreference from location to the root package since it is
now used in several places.

Add user action now uses DimmableIconPreference. Added a new summary string,
which is displayed when no more users can be added.

Bug: 20892920
Change-Id: I00b00f80ba8933a00a2de85777b9f7e55d03c31b
6 files changed