Install android_target into pants and add example app for integration tests

The icon is from the Android SDK under the Apache license. I can see a world where a Pants icon would be
preferred. I am comfortable putting the Pants license on everything else as I mostly wrote it by hand.

For the install into Pants, to successfully run pants, backend.android:plugin needed to be in the
pants/bin/BUILD. However, for the testing to also successfully run, it needed to be added to
pants/base:dev_backend_loader.

Testing: ci.sh

                         ===== 9 passed, 2 skipped in 101.36 seconds ======
                     
    14:39:49 01:43     [junit]
    14:39:49 01:43     [specs]
                   SUCCESS

    [== CI SUCCESS ==]