Tweak RateEstimator's alpha value

The original value of 0.8 was not sensitive enough for blocking misbehaving apps, but 0.5 broke a test that updates notifications rapidly. Let's settle for something in the middle.

Test: atest CaptivePortalLoginTests:com.android.captiveportallogin.DownloadServiceTest --iterations 5
Bug: 280098361
Fixes: 285559860
Change-Id: I85dbdf49684d58e20b6032a83a5da24d1399bd3c