Fix the number of levels returned by getLevels().

This was previously out of sync with WifiManager.RSSI_LEVELS which
caused UI issues where the wrong number of bars were shown in various
places. This is the permanent fix to a previous temporary fix.

Bug: 1948619
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java

Change-Id: Iadb2a6131ee9fbc4f297766d0bc3bc82eca40a4d