Allow WifiSettings to show old APs on app resume.

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
Merged-In: I4f9b2ec855e057e28235b0253ab42c6b4521bebc