The intention of this test case is to check that the cached hotspot prefix doesn't get assigned to ethernet, even if released, so that it can be offered to the hotspot if requested again.
Therefore we should call `requestStickyDownstreamAddress` instead of `requestDownstreamAddress` when requesting the prefix for hotspot.
Bug: 375858174
Test: atest TetheringTests:com.android.networkstack.tethering.PrivateAddressCoordinatorTest#testReservedPrefix --iteration 200
Change-Id: I5d897aa958a9549f3d773bd59920906a815edb5e
27eed97a14