Move the test template to //testing/test.gni (part 2)

This CL move the test target from build/config/BUILDCONFIG.gn to testing/test.gni

It also update the test template to automatically build an apk on Android.

R=cjhopman@chromium.org,brettw@chromium.org

Review URL: https://codereview.chromium.org/824263004

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