test_runner.py: Detect supported ABIs from --additional-apks

Found this was necessary for system_webview_shell_layout_test_apk, which
doesn't have any native code, but the underlying webview apk does.

Bug: 853280
Change-Id: I6a3559d41309c22c1cc4027f7387b619d04b7e4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627935
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662834}

build/android/pylib/instrumentation/instrumentation_test_instance.py

@@ -834,10 +834,15 @@ class InstrumentationTestInstance(test_instance.TestInstance):
 
   #override
   def GetPreferredAbis(self):
-    ret = self._test_apk.GetAbis()
-    if not ret and self._apk_under_test:
-      ret = self._apk_under_test.GetAbis()
-    return ret
+    # We could alternatively take the intersection of what they all support,
+    # but it should never be the case that they support different things.
+    apks = [self._test_apk, self._apk_under_test] + self._additional_apks
+    for apk in apks:
+      if apk:
+        ret = apk.GetAbis()
+        if ret:
+          return ret
+    return []
 
   #override
   def SetUp(self):