Add individual ScanResult speeds to verbose logging.

Moves the individual ScanResult summary string generation out of
getVisibilityStatus into its own method `verboseScanResultSummary`,
to reduce duplication and to enable testing the per-ScanResult speed labeling.

Bug: 63866500
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java

Change-Id: I9583d142a7d50314017154703df4e1ce69fc68aa