Refer the SoftApCapability cache if getUsableChannels is unsupported

- Get SAP band capability from SoftApCapability#getSupportedChannelList

Bug: 272450463
Bug: 338503507
Bug: 340789513
Flag: EXEMPT bugfix
Test: Manual testing
atest -c WifiHotspotRepositoryTest

Change-Id: I7b792c27bc3c41e78417032f797d57b030d6db72