fix install android debug samples apk error.

[suzp1984]

cp not exist debug sample apk.

./build.sh -p android -k sample-gltf-viewer -i debug Take sample-gltf-viewer as an example, above cmd will result in :

cp: samples/sample-gltf-viewer/build/outputs/apk/debug/sample-gltf-viewer-debug-unsigned.apk: No such file or directory

The root cause is that the android will signed the debug apk with ~/.android/debug.keystore, which is the default generated by android gradle plugin globally.

remove unsigned tag from release apk don't make sense

[Cause] the unsigned apk will not installed on the android, the apk must be signed anyway. So I think remove the unsigned keyword don't make sense, at lease let people know, it's an unsigned apk.