Listen to RSSI_CHANGED_ACTION broadcasts in WifiTracker.

This solves issues where the phone status bar and connected AP in the
picker are out of date.

Bug: 35971425
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/and
roid/settingslib/wifi/WifiTrackerTest.java

Change-Id: I2a7c50de6c754623fd262aac4e9e21d6bd7fc5cf