Incorporate home metadata into app details UI

Just as we don't expose disable/uninstall of home applications in this UI,
we now also don't allow disable/uninstall of packages that are proxying
for home apps via the android.app.home.alternate meta-data mechanism.

Also, don't display the 'Home' settings top-level category at all when
there is only a single available home app.

Finally, explicitly note the current user when sending broadcasts,
otherwise API sanity checks get suspicious.

Bug 10749961

Change-Id: I13e11032cb571df19798c4e13c91a572d1ee61a7