Refactor WifiTracker sStaleScanResults.

In ag/2580164, in order to minimize changes, we used mutable static
state in WifiTracker to control cache eviction. This breaks a certain
flow in SetupWizard. This change reverts sStaleScanResults to an
instance member and then pipes the value into the AccessPoint update
call to control cache eviction instead of relying on error prone use of
mutable static state.

Bug: 63479352
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java
&&
runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java
Manual: See bug

Change-Id: Ia6d79c1904294da69841cbdf6eafbf42fa70f2d0