Mobile device vendors develop their customized Android OS (termed downstream), such as MiUI and LineageOS, based on Google Android (termed upstream) to support new features. During daily independent development, the downstream also periodically merges changes of a new release from the upstream into their development branches, keeping in sync with the upstream. Due to a large number of commits to be merged, heavy code conflicts (named textual conflicts would be reported if auto-merge operations failed. Prior work has studied conflicts due to such merge commits. However, it is still unclear about the coupling between the downstream and the upstream (We term this coupling as the dependency facade), as well as how merge conflicts are related to this coupling. To reveal and characterize them, we first propose the DepFCD to reveal the dependency facade from three aspects, including interface-level dependencies that indicate a clear design boundary, intrusion-level dependencies due to direct modifications made by the downstream to the upstream, and dependency constraints imposed by the upstream non-SDK restrictions. We then empirically investigate these three aspects (RQ1, RQ2, RQ3) and merge conflicts RQ4) on the dependency facade. To support the study, we collect four open-source downstream projects and one industrial project, with 15 downstream and 15 corresponding upstream versions. Our study reveals several interesting observations and suggests earlier mitigation of merge conflicts through a well-managed dependency facade, such as monitoring dependency changes and detecting the violations of dependency constraints. Industrial experts responsible for the studied industrial project have confirmed our observations and taken our suggestions. Our study will benefit the research about the coupling between upstream and downstream as well as the downstream maintenance practice.