Android Studio: Update docs

Instrumentation code is only available for editing purposes.

TBR=estevenson@chromium.org

Bug: 620034
Change-Id: I32d2b3b29bd61a2b206f6ccca80cfb9cde71a292
Reviewed-on: https://chromium-review.googlesource.com/1099177
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566898}

docs/android_studio.md

@@ -181,8 +181,9 @@ resources, native libraries, etc.
 
 * Android Studio v3.0-v3.2.
 * Java editing.
+    * Application code in `main` sourceset.
+    * Instrumentation test code in `androidTest` sourceset.
 * Native code editing (experimental).
-* Instrumentation tests included as androidTest.
 * Symlinks to existing .so files in jniLibs (doesn't generate them).
 * Editing resource xml files
 * Layout editor (limited functionality).