Average scan results for AccessPoints.

This reduces jitter in the WifiTracker and is modeled after a prior
broken implementation which attempted to half-life the rssi each time
AccessPoint.update(ScanResult) is called.

Bug: 34774783
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java

Change-Id: Iab3f9914af40f2fd56e8ae7b45dcbd62176c8a67