Various work on out of memory managment.

- Improve how we handle processes that have shown UI, to take care
  of more cases where we want to push them into the background LRU
  list.
- New trim memory level for when an application that has done UI
  is no longer visible to the user.
- Add APIs to get new trim memory callback.
- Add a host of new bind flags to tweak how the system will adjust
  the OOM level of the target process.

Change-Id: I23ba354112f411a9f8773a67426b4dff85fa2439