Here's a successful method to scroll in Android.
element_to_tap = self.driver.find_element_by_xpath(<xpath_to_element_near_bottom_of_screen>)
element_to_drag_to = self.driver.find_element_by_xpath(<xpath_to_element_near_top_of_screen>)
All you need to do is feed it two webdriverobjects, it seems to do the rest. But I haven't fully validated it works in all of my scenarios, that it can scroll down as well as up, that it doesn't over-scroll, etc.
It doesn't look like Android will ever provide page_source that contains out-of-view elements, including after you scroll (items at the top of the page are now out-of-view after a scroll and are dropped from XML). So there's going to have to be some pretty clever functions coded to support all of the various scrolling I will need to do.
For example, many of my tests need to count the number of rows
//android.widget.ListView/android.widget.LinearLayout in my view to make sure it matches summary counts elsewhere in the app. I'm going to need to scroll, count up the visible rows, scroll more, count up the visible rows making sure not to count ones already counted (guess I'll have to keep track of some hopefully visible unique identifier in each row), and stop counting once the list has beens scrolled (again keeping track of hopefully visible unique identifier and stop scrolling once no new uniques appear after a scroll). It all seems overly complicated. I'd think by now these would be basic functions provided by Appium.
I'm hoping someone with more experience will chime in with the "best" way to do this.