Fix AccessPointPreferenceTest#refresh_openNetwork_updateContentDescription

AccessPoint now uses
WifiManager#calculateSignalLevel(rssi) instead of
WifiManager#calculateSignalLevel(rssi, numLevels).

WifiManager#calculateSignalLevel(rssi, numLevels)
previously redirected to
ShadowWifiManager#calculateSignalLevel(rssi, numLevels),
which always returned numLevels - 1 = 5 - 1 = 4.

WifiManager#calculateSignalLevel(rssi) is not
redirected by ShadowWifiManager, so it returns 0.
Fixed by updating the expected value of the assert.

Bug: 147703223
Test: make RunSettingsLibRoboTests -j40
Change-Id: Ief86fde8ced462e9e9cbb75af0b95c2f7ab26cdf