WifiNetworkFactory: Increase scan cache age to 30 seconds

Since we perform periodic scans every 10 seconds, using 20 second as
scan expiry will not always include scan results from last 2 scans
in the cache.

Bug: 169771656
Test: atest com.android.server.wifi
Change-Id: Ic3c100dbd9cb27ade84c435ebcbffab07cee3366
1 file changed