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: Ida0955393831bf74b449bbf89aec5cb6447394d5
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: I842a9bf7b39cd469546f842e96782e4acb44de04