Fix mojo_android_application when there is no java sources.
Fix mojo_android_application when there is no java sources.

R=ppi@chromium.org

Review URL: https://codereview.chromium.org/1227953007 .

afandria/mojo,jianglu/mojo,jianglu/mojo,jianglu/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo