Add Tetheroffload AIDL interface support

Add the AIDL HAL service interface support and abstract the HIDL/AIDL
interface implementations to allow OffloadController to talk to both
interfaces.

Bug: b/205762647
Test: atest com.android.networkstack.tethering
      atest ConnectivityCoverageTests
      atest ConntrackSocketTest
Change-Id: If7634a298d03668ed70b328f1ad6ca0ac0fe170b