Add an option to sign .apk in qt_android_add_apk_target.

androiddeployqt expects either --sign option or QT_ANDROID_KEYSTORE_* environment vriables.

To date it is impossible to setup a signing environment during a build, i.e. env vars should be setup system/user wide or a wrapper script should be used.

    add_custom_target(${target}_make_apk
        DEPENDS ${target}_prepare_apk_dir
        COMMAND  ${deployment_tool}
            --input ${deployment_file}
            --output ${apk_dir}
            --apk ${apk_dir}/${target}.apk
        COMMENT "Creating APK for ${target}"
    )

Adding the possibility to pass somehow a signing information into qt_android_add_apk_target on the build step would be very useful.