Allow system apps to substitute a different app name in notifications.

This is a privileged permission and is only to be used by
the core OS and related packages whose names are confusing
or misleading when shown in notifications.

The user will always be able to see the true package name by
accessing the notification inspector (longpress or swipe
gesture on the notification row in SystemUI).

Fixes: 26517701
Change-Id: I2b021c9da0757b99df76399666af263668d88070