[Mac Randomization] Fix MAC randomization option does not work when

ephemeral newtork

WifiConfiguration is only available when saved network. So ephemeral
newtork could not have wifi config to config wifi setting. In this
situation, making option be not selectable.

Bug: 122919564
Test: atest WifiPrivacyPreferenceControllerTest
Change-Id: I8a2a1c36ec6d41a6f912da29a8b17bc2e43d9a27