Add SLOW network and remove deprecated enums.

Stop using deprecated API enums and instead define internal constants
for different network speeds. Add support for 'SLOW' networks.

Bug: b/35628911
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java

Change-Id: I3394f801118adffcb4ba49035e0b22eebb9f0e9a