Allow PhysicalChannelConfig.build without setBand for unknown band

Allow building a PhysicalChannelConfig without setBand() if the
band is unknown, It should be enforced not to allow "unknown"
bands in the near future.

Bug: 288310456
Test: atest FrameworksTelephonyTests & tested on live network
Change-Id: I1bcdeafcb5e78859985bb100c81ad0acf88db0c2