[SysApps] Introduce the SysappsManager

This class will be the single entry point for registering all
the SysApps extensions. It should be the only class of the "sysapps"
namespace exposed to other subsystems.

In the future it will also manage the lifecycle of data providers (platform
abstractions for extensions like DeviceCapabilities).

This branch also adds a master switch for disabling SysApps as a whole.