Fix path to constants.gradle

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=164095350

demos/main/build.gradle

@@ -11,7 +11,7 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-apply from: '../constants.gradle'
+apply from: '../../constants.gradle'
 apply plugin: 'com.android.application'
 
 android {