EthernetTetheringTest: obtain the required permissions when needed

Don't adopt all permission in setup, instead obtain permission when
needed. It helps to avoid unexpected permission dropping in functions.

Bug: 237369591
Bug: 242067530

Test: atest EthernetTetheringTest

Change-Id: I1215278271ca6b48e097d7f2cf160416d5411655
2 files changed