Fix test_hotspot_upstream_cellular failure

This change:
  1. Move precondition checks prior to setup hotspot.
     This removes the need of redundant clean-up if
     the device doesn't meet preconditions.
  2. Use unregisterAll instead of unregisterCell to
     clean-up registered connections to prevent from
     complex error handling in clean-up stage.

Test: m connectivity_multi_devices_snippet && \
      atest CtsConnectivityMultiDevicesTestCases:ConnectivityMultiDevicesTest
Fix: 345110845
Change-Id: I4de0e7401d52f891948eb7b5a8c112a837cb0c3e
3 files changed