base

Start combining threads in system process.

This introduces four generic thread that services can
use in the system process:

- Background: part of the framework for all processes, for
work that is purely background (no timing constraint).
- UI: for time-critical display of UI.
- Foreground: normal foreground work.
- IO: performing IO operations.

I went through and moved services into these threads in the
places I felt relatively comfortable about understanding what
they are doing.  There are still a bunch more we need to look
at -- lots of networking stuff left, 3 or so different native
daemon connectors which I didn't know how much would block,
audio stuff, etc.

Also updated Watchdog to be aware of and check these new
threads, with a new API for other threads to also participate
in this checking.

Change-Id: Ie2f11061cebde5f018d7383b3a910fbbd11d5e11