Move code for handling uid obs "cutoff" to activity manager

To do this, I had to fix the PROCESS_STATE_NONEXISTENT constant
to be the last value (instead of the special magical -1 value)
so it semantically matches the public importance constants.  I
think this is better anyway.

Also this fixes a big problem in the implementation, where we
weren't keeping track of the last proc state per uid...!  Duh.

Test: manually ran testUidImportanceListener
Change-Id: Ie3008f824446089840f896885e6033472abb065e