Annotating ethernet network management APIs in EthernetManager with @SystemApi. Bug: 210485380 Test: atest EthernetServiceTests atest CtsNetTestCasesLatestSdk Change-Id: I10429441fd4d7b9bcaa7437b844420a43a415d72 Merged-In: I10429441fd4d7b9bcaa7437b844420a43a415d72