Stop using adoptShellPermissionIdentity in setUp

Using adoptShellPermissionIdentity in setUp to use the shell permission
idnetity in whole test make the test not easy to
development. If the test want to use the library which would also
start/stop shell permission identity, this let the test lose shell
identify unexpectly after using that library. Only adopt shell permission
idnetity when necessary instead.

Test: atest CtsTetheringTest
Change-Id: I6cf24e6cbf227944bc914446337a247dc350562a