Update package info warning message to indicate it could be noise

I got a frightening-looking "unable to find package info" warning
on my first attempt running Android tests. The docs
(//android/javatests/.../vr/README.md) mention that this warning
can occur on first run. This CL updates the warning message to
indicate that it can be noisy. (In the end, the tests ran with
no problem.)

Change-Id: Ia1311051614204a5179757de4207c81a6a3060d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023388
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735589}

build/android/pylib/instrumentation/instrumentation_test_instance.py

@@ -616,7 +616,10 @@ class InstrumentationTestInstance(test_instance.TestInstance):
           self._package_info = package_info
           break
     if not self._package_info:
-      logging.warning('Unable to find package info for %s', self._test_package)
+      logging.warning(("Unable to find package info for %s. " +
+                       "(This may just mean that the test package is" +
+                       "currently being installed.)"),
+                       self._test_package)
 
     for apk in args.additional_apks:
       if not os.path.exists(apk):