Always evict old scanResults(> 15s) in WifiTracker

When fragment is created, it will force display all the APs
from mWifiManager.getScanResults, where it may have obsolete APs.

This CL will evict old scanResults even in this force display, so
it won't show obsolete APs anymore.

Also change the buffer size from 25s to 15s so it only display APs
scanned in last 15s.

Bug: 111738605
Test: SettingsUnitTests
Change-Id: I9413bcba81dba55c6b13cfdca2785069d4d7d105