Add atom for sdk extension tests

This atom will be logged from a test app in /product/app. It is
temporary, and will be removed after successful R test missions.

Bug: 148945438
Test: out/host/linux-x86/bin/statsd_testdrive \
          -p com.google.android.apps.internal.sdkext.testapp 354
Change-Id: Ic25d601bfdb6f84cb9963581213ee6d3300424ab