vndk-def: Add VTool prototype in vndk_definition_tool

Using the command "dep-graph" the tool will check and show the VNDK
dependency violations base on the VNDK tag definition.

Test: Run `vndk_definition_tool.py dep-graph \
        --system ... --vendor ... -o "html_output" \
	--tag-file "[vndk_tag_table].csv"`

Change-Id: Ia5196e94beb0de106176e98ea413e4a921d7f8c5