Allow WifiSettings to show old APs on app resume.

MANUAL MERGE of ag/2398149 to avoid merge conflicts when trying to
submit to oc-dev.

If we do not have any recent scan results, show the previous APs from
when the app was last paused, by not triggering a force update.

Accompanying changes are also made in WifiTracker to prevent
onAccessPointsChanged callbacks from being invoked until the tracker
receives a new 'SCAN_RESULTS_AVAILABLE' broadcast (ag/2409026).

Bug: b/38212080
Test: runtest --path
packages/apps/Settings/tests/unit/src/com/android/settings/wifi/WifiSettingsUiTest.java
one minute later.

Change-Id: I4f9b2ec855e057e28235b0253ab42c6b4521bebc