Add new proc state constants and delivery.

The activity manager now keeps a new "process state" for
each process, indicating the general execution and memory
state of the process.  This closely follows the out-of-memory
adjustment and scheduling class that it currently tracks,
but roles these together (plus a little more info) into one
more semantically meaningful number.

This value is reported to each process as it changes, so they
can do things like tune the Dalvik garbage collector to match
the current process state.

I think I should also switch to this for process states.  It
will give is more meaningful divisions of time for each process.

Also fix a problem in the activity stack where the previous
process was not being set correctly when moving between
activity stacks.

Change-Id: I598b1667dc46547f8fadae304e210c352cc9d41f