Update WIFI_CONNECTION_STRENGTH according to Wi-Fi signal split level

Update WIFI_CONNECTION_STRENGTH in AccessPointPreference to have
correct ContentDescription according to split level of Wi-Fi signal
strength.

Bug: 62182689
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java

Change-Id: Ic06601ccda3d81c30c694c3e4070e26c98bb110b
Signed-off-by: Masafumi Hasegawa <masafumi.x.hasegawa@sonymobile.com>