I keep my vault in Dropbox and have tried several note apps on Android.
I found Simple Markdown which is free and has a Markdown and Preview panes and will open and save to Dropbox (assuming you have the Dropbox app previously installed on your Android phone)
I’ve used it for a few days now on my Samsung Note 4 running Marshmallow and it seems very reliable for basically pulling up a markdown file, editing/previewing the content and saving it back to the vault on Dropbox
Interestingly this app is open source so it could be forked and made more Obsidian markdown aware and have links open up new files by some enthusiastic developer light_smile: