Allow more background services

* Some services or broadcast may get tremendous delay because
  of the limited background services. It will bring about many
  issues like alarmmanagerservice's wakelock held too long.

(from CAF)

Change-Id: I39f54a9c23256cecfa6f0e9b08f1705049e031ea
Set background app limit to 12

* For low memory devices, the original limit of 24 would be
  impractical anyways since it cannot contain that many apps.

(from CAF)

Change-Id: Id2073779c01d2c21b5397a5fd7b0fcda7f5e2dfa