Convert Tetheroffload HIDL to AIDL.

This patch converts Tetheroffload HIDL to AIDL and adds an empty
example implementation.
There are some changes in AIDL HAL:
1. Combine IOffloadConfig and IOffloadControl into IOffload
2. Use AIDL builtin errors exception code with message as return

Bug: b/205762647
Test: m android.hardware.tetheroffload-update-api
      m android.hardware.tetheroffload-V1-ndk
      m android.hardware.tetheroffload-service.example
      atest VtsHalTetheroffloadTargetTest
Change-Id: I9859ff3df608c058786b4e2b91cb6cd5f38e653a