Support loading configs from APEX

When wpa_supplicant is in its APEX, it loads configs (overlay/template)
from the APEX. It gets the APEX path from the executable's path.

Bug: 325392832
Test: atest CtsWifiTestCases
Change-Id: Idabbe75cca1d6d9ec987053525ef3b81d2ca6bda
1 file changed