When merging into Android, add tests/Android.mk.
When merging into Android, add tests/Android.mk.

Once https://codereview.chromium.org/235883015/ is submitted, we will
be generating tests/Android.mk in gyp_to_android.py. Add it during the
merge.

BUG=skia:2447
R=borenet@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/254963004

Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot