Android: Document unit test runner incompatibility with recent versions

Android unit tests don't work on Android R+, but this isn't mentioned
in the docs, and the error message is pretty arcane and hard to
understand. This change updates the docs to mention this problem.

Bug: 1094062
Change-Id: I77b196febde1009eb13b195be937243e96ace1a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495968
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820493}

docs/testing/android_test_instructions.md

@@ -130,6 +130,12 @@ resize2fs android_emulator_sdk/sdk/system-images/android-25/x86/userdata.img 1G
 tune2fs -e continue android_emulator_sdk/sdk/system-images/android-25/x86/userdata.img
 ```
 
+### AdbCommandFailedError: failed to stat remote object
+
+There's a known issue (https://crbug.com/1094062) where the unit test binaries can fail on
+Android R and later: if you see this error, try rerunning on an Android version
+with API level <= 29 (Android <= Q).
+
 ## Symbolizing Crashes
 
 Crash stacks are logged and can be viewed using `adb logcat`. To symbolize the