Doesn't it mean that you have to use LockOSThread() for your main thread in order to avoid messing up the Android event loop, negating the advantages of cheap threads and forcing you to eat context switches when calling into Java land?
No, the main go thread will receive events on a channel, all GUI calls can be wrapped channel writes, which get sent to the Android thread. Like how React Native is modeled.
In that case you're paying the cost of even more context switches. One to get into the UI thread and one to get out, in the case of a synchronous call. In a LockOSThread solution you don't need any context switches. You also save the memory cost of one goroutine (which is admittedly small).
