Most likely its the undefined ACL causing your access problem ( entity in back end not being retreived by a GET ).
So, you should read the section of the android manual on ACL and on security. As a starter, when you create an object , you should construct an ACL with READ&WRITE for the owner and READ for others...
In android granting either world read/write or exclusive read/write to owner , that would be :
aclOb1 = new ObjectMapper().createObjectNode();
aclOb2 = new ObjectMapper().createObjectNode();
aclOb2.put("read", true);
aclOb2.put("write", true);
String oid = $Get.current.user
if(oid.equalsIgnoreCase("default")){
aclOb1.put("*", aclOb2);
}else{
aclOb1.put(oid, aclOb2);
}
rootOb.put("ACL", aclOb1);
For ACL example in JS , see createOnEnter method in the link