[Wi-Fi] Apply cache in WifiTracker to update the signal icon immediatelly.

issue: Signal icon on the add wifi network feature need to wait for the scan result.
Solution: Apply the cache in the WifiTracker to update signal icon for
shortening the waiting time of new scan result.

Bug: 146842198
Test: Add following unit test cases to test signal level has been updated correctly:
      1. withOneSuggestion_whenScanResultChangedButWifiOff_uiListShouldHaveZeroLevel.

Change-Id: Id72e2b43e020f2cafa2af4af5ffb1c28529002bd