Usage stats!

Start reworking the usage stats service to be able
to have an API we can publish.

The basic information it keeps is still the same, though
that will be changing in the future.  The one big addition
here is that we are also now collecting configuration usage
stats.

Also introduce the start of an access model for usage stats,
using app ops.  There is an new app op that gives an application
access to usage stats even if it normally wouldn't have it,
disabled by default.

Change-Id: I6ead28e18a7f08eafd057d6ff37dd9cb216358f4