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)

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: Ibc679513c44810393f015afc7fb04fe2563dc413