A new attribute which can show the dynamic summary

For now, slice view shows screen title by default,
but it isn't approprate for some simple cases.
ie, device model, phone number, android version etc.

So, We create a new attribue which let BasePreferenceController
be more flxible. User can choose what they want to show
in summary text.

Fixes: 74900516
Test: make RunSettingsRoboTests

Change-Id: I2788c6edfaf8e656170a507607f22513841a3e0a