Add /system_ext/etc to config search path

/product should be at the highest search order.
Load libnfc-nci.conf from /system or /system_ext partition if it is not
found in /product or other partitions.

Bug: 157704723
Test: adb push libnfc-nci.conf to /system_ext and check if it is loaded
Change-Id: Ic6a9399497d20ba0bff468d043ce3b5e04453aa8
Merged-In: Ic6a9399497d20ba0bff468d043ce3b5e04453aa8
(cherry picked from commit 707ff2b12514e3bd712066ddaa4b3982ba9c1abc)