Do not evict scan results in cold start.

When starting WifiTracker, show whatever results are available from
platform until new scan results come in. This allows us to show the best
available results from the previous location scan. Once we have resumed
scanning and new scan results come in, the previous logic of evicting
scan results older than 15 seconds applies.

Also prevent unnecessary onConnectedChanged callbacks from being fired,
which may unnecessarily cause UI reloading bars to appear by performing
unnecessary work.

Bug: b/38212080
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

Change-Id: I3db9d98425b3d2fc66fa0757807a318d0c7456ee