custom: added APN config files to prebuilt

Devices needs to be changed to use somethling like this
e.g. custom_find5.mk

PRODUCT_COPY_FILES := vendor/custom/prebuilt/etc/apns-conf.xml:system/etc/apns-conf.xml

Change-Id: Id6d84a1698b7c4e6e58170c93cd6a61e49bda966
Merge "custom: added APN config files to prebuilt" into android-4.3