Add PC feature flag

This CL adds a feature flags for traditional clamshell and desktop
computing devices. It can be used by apps to specifically target
those device types (such as Chromebooks) and by the framework to
recognize apps that have been designed for that form-factor.

Bug: 37359238
Test: None
Change-Id: Ie10443583997bcf1bff0b44da31af3f144f7aa0b