Update detail icon only if its signal level changed.

Changes behavior to only render the wifi pie icon if the new rssi has
resulted in a new signal level.

Bug: 64712237
Test: make -j40 ROBOTEST_FILTER=WifiDetailPreferenceControllerTest
RunSettingsRoboTests

Change-Id: I01be2f87d61489242d646ff2003db13c9268ea14