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
diff --git a/tetheroffload/aidl/vts/functional/Android.bp b/tetheroffload/aidl/vts/functional/Android.bp
new file mode 100644
index 0000000..c9c1845
--- /dev/null
+++ b/tetheroffload/aidl/vts/functional/Android.bp
@@ -0,0 +1,24 @@
+package {
+    default_applicable_licenses: ["Android-Apache-2.0"],
+}
+
+cc_test {
+    name: "VtsHalTetheroffloadTargetTest",
+    defaults: [
+        "VtsHalTargetTestDefaults",
+        "use_libaidlvintf_gtest_helper_static",
+    ],
+    srcs: [
+        "VtsHalTetheroffloadTargetTest.cpp",
+    ],
+    shared_libs: [
+        "libbinder_ndk",
+    ],
+    static_libs: [
+        "android.hardware.tetheroffload-V1-ndk",
+        "libgmock_ndk",
+    ],
+    test_suites: [
+        "vts",
+    ],
+}