wifi: get the correct mainline version when wifi is disabled

update getWifiStackVersion() to return the correct mainline version
even when wifi is disabled.

Bug: 166835054
Test: atest com.android.server.wifi
Test: manual test to confirm post-boot detection can detect SW version
change correctly

Change-Id: I3eb878145a761452ddcfdd0b5c424e78212fc799
Merged-In: I3eb878145a761452ddcfdd0b5c424e78212fc799
2 files changed