WifiManager: Promote EXTRA_WIFI_AP_FAILURE_REASON to @SystemApi

EXTRA_WIFI_AP_FAILURE_REASON is @hide, but used
by Android Auto for debugging purposes. Promote
it to @SystemApi.

Bug: 143970861
Test: compiles
Change-Id: I427ea9a2af0c74cb3edfb84fbb2305cd76355185