Add background app limit tunable.

Set MAX_CACHED_APPS based on a system property "ro.sys.fw.bg_apps_limit"
so that number of background apps can be tuned accordingly.

Conflicts:
        services/core/java/com/android/server/am/ProcessList.java

Change-Id: Ie21b0961dad10e1cd6da3cfe2a45eff45dc8756d