SDK only: now that support lib is in SDK, we can link to it.

Update some of the platform documentation to directly link to
relevent support lib docs.  Yay!

Also improve BroadcastReceiver documentation to more clearly
discussion security around receivers, and how the support
lib's LocalBroadcastManager can help.

Change-Id: I563c7516d5fbf91ab884c86bc411aff726249e42