Refactor ProcessStats.

- Pull most of the inner classes out into their own files.
- Move everything to a new android.app.procstats package.
- Move all of the code that was manipulating the big list
  of longs to use the new SparseMappingTable class rather
  than doing everything by hand. The logic is unchanged.
- Add a sequence number check to SparseMappingTable so
  when the big list of longs and the individual tables are
  reset, which happens somewhat independently, we can
  assert when one of them doesn't get reset.