Refine WifiHotspotRepository variable names

- Since the speed of hotspot design was changed to refer to allowed channels rather than usable channels, remove the "Usable" naming to avoid confusion

Fix: 360312024
Flag: EXEMPT refactor
Test: Manual testing
atest -c WifiHotspotRepositoryTest

Change-Id: Ie54a351e0f788e1b6aeec079ccf6671f7f78fb1c