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/default/tetheroffload-example.xml b/tetheroffload/aidl/default/tetheroffload-example.xml
new file mode 100644
index 0000000..9fe83f6
--- /dev/null
+++ b/tetheroffload/aidl/default/tetheroffload-example.xml
@@ -0,0 +1,7 @@
+<manifest version="1.0" type="device">
+    <hal format="aidl">
+        <name>android.hardware.tetheroffload</name>
+        <version>1</version>
+        <fqname>IOffload/default</fqname>
+    </hal>
+</manifest>