Add safeguard against previous ethernet interfaces existing

The test EthernetManagerTest#testEnableDisableInterface_callbacks
captures the interface state changes of ethernet
interfaces that already exist instead of the test one created. Changes
the expectCallback to eventuallyExpect to account for this.

Test: atest EthernetManagerTest
Flag: TEST_ONLY
Bug: 333338569
Change-Id: Ic08444c87f93b3832c4fae1ce2f8382bbb71cb28
1 file changed