Go to File>Preferences>Editor>File Types then select Files Opened In AssociatedApplications and add *.md to the list
Reassign the wild card
Do likewise for the Text file type
Enable the Markdown
Go with Vladimir Schneider
Preview like a bawse!
UPDATE: As mentioned by @desgraci below. It's now a paid plugin. It doesn't even show up. It would ideally work as indicated in the updated documentation but some requests left in the reviews.
A little late but you can go to Settings/Preferences -> Editor -> File Types. Select Markdown Language from the list. Below, in the Registered Patterns section, add *.md, press ok, and restart Android Studio
There is another plugin which is "Markdown Navigator Enhanced" and that I tested on Android Studio 3.5.2, it enables a toolbar with some editing functions as well as the preview options.
You can Edit, preview or do both in a split window, you can even show the HTML code.
The plug-in is paid, but it has a 30-days trial that can be good for a short usage.
This is not just a JavaFX issue, which can be resolved by installing Choose Runtime plugin and choosing a different JDK. The Markdown plugin itself also references classes from IntelliJ UI, which means that even with a runtime that includes JavaFX, it still requires IntelliJ and won't work properly with Android Studio:
Caused by: java.lang.ClassNotFoundException: com.intellij.ui.javafx.JavaFxHtmlPanel PluginClassLoader[org.intellij.plugins.markdown, 193.6494.42] com.intellij.ide.plugins.cl.PluginClassLoader@4fa7c8ac
at com.intellij.ide.plugins.cl.PluginClassLoader.loadClass(PluginClassLoader.java:77)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
... 93 more
It will still work with the default bundled JDK, just reduced functionality without the preview panel.