Check process state when killing background processes for density

Previously, we were not killing many processes because we weren't in an
actual OOM condition. This CL moves the background killing rules to a
separate method and checks the process state. This removes the OOM
adjustment check and *will* kill persistent processes.

Bug: 27397233
Change-Id: Ibc21e97cfc729a0835e613095acd9a4b648906d8