Sort APs by Speed label value instead of ranking score.

This prevents unintuitive sorting if the ranking score (somehow)
sorts differently from the speed label.

This change also fixes a broken test in AccessPointTest.java
'testSummaryString_showsSpeedLabel()'.

Bug: 63116984
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java

Change-Id: I4a9e55950541ddbf32e01508484bd40326ef228e