Introduce SystemUI-managed alternative system bars.

If a service component is defined in a new secure setting,
SystemUI will attempt to use that service as the status bar
provider.

Falls back to the existing in-process implementation configured
in the product config if the setting is missing or invalid.

Nothing changes yet from a permission point of view.  Alternative
system bar implementations still require the status bar permission.

Also nothing changes from an api point of view.  Alternative
system bar implementations use the existing IStatusBar interface.

This simply enables testing alternative system bar implementations
installed from other trusted, platform-signed packages.

Known caveat: the setting is stored per user, multi-user changes
will be handled in a future CL.

Change-Id: I0413df185f7e75f77ad2ae1bc3689306d5e6e0fb