Do not pause scanning during Network state changes.

This prevents UI bugs in WifiSettings where scan results would become to
old upon switching networks and thus all the APs would be removed from
the wifi picker. Now scanning is only paused/resumed during start/stop
tracking as well as on wifi state changes between enabled/ not enabled.

Also adds additional logging to scan result age in verbose logging.

Bug: b/64989100
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java
Manual: 1. Repeated switch back and forth between different networks.
2. Ensure that AccessPoints in WifiSettings do not disappear after new
connections.

Change-Id: I11e424bcce799a5f3d003a955dcee004294058b7