Add methods to indicate scanning status

Add onScanRequested() and onWifiEntriesChanged(int reason) callbacks to
indicate scanning state. This is useful for Settings to show the
progress bar while the device is scanning.

Bug: 308567866
Test: atest WifiTrackerLibTests
Change-Id: I9b92e52195445dea915e7c77d95f98d7ceb23465
3 files changed