Clear AccessPoints in WifiTracker when Wifi is Off.

When Wifi is not Enabled, we should clear existing access points. This
avoids showing stale data when wifi is turned back on. Since this method can
now invoke onAccessPointsChanged callbacks, add extra gating for
mStaleScanResults.

Improve test flakiness by waiting for Work and Main handlers to
finish processing messages before continuing.

Bug: b/37832771
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java

Change-Id: I34faabec01a7188debe3c3b2fe6a0e02f49b320f