Use [clear|restore]CallingIdentity instead of withCleanCallingIdentity

ConnectivityService is going to be a part of mainline module, to
prevent using @hide method - withCleanCallingIdentity(), use
clearCallingIdentity() & restoreCallingIdentity() instead.

Bug: 170598012
Test: FrameworksNetTests & CtsNetTestCasesLatestSdk
      & TetheringIntegrationTests
Change-Id: I20f45af4a4044844a171b09269981451a9d7cdb4