[android] Handle missing logcat file while running tests.

TBR=mikecase@chromium.org,hzl@chromium.org

Change-Id: Id7187e1fa622103d5a8e02f4404d42a5ebcd6a5b
Reviewed-on: https://chromium-review.googlesource.com/529976


Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478526}