[Wi-Fi] Add Wifitracker mechanism to update the signal icon.

Signal icon on the add wifi network feature is constant, didn't change with real signal level.

Bug: 146842198
Test: Add following unit test cases:
      1. withOneSuggestion_uiElementShouldHaveInitLevel() to test init level.
      2. withOneSuggestion_whenScanResultChanged_uiListShouldHaveNewLevel() to test level should updated according to scan result.

Change-Id: I212a825da9455a09a5c7777f87c0a74ec030c890