Add setter setUseCaseId

Allows better flexibility when creating the button programatically.
Otherwise the only other way to set the useCaseId would be to create a custom `AttributeSet`.

Author: gausam

yoti-sdk/src/main/java/com/yoti/mobile/android/sdk/YotiSDKButton.java

@@ -70,6 +70,10 @@ public void init(AttributeSet attrs) {
         setOnClickListener(this);
     }
 
+    public void setUseCaseId(String useCaseId) {
+        mUseCaseId = useCaseId;
+    }
+    
     private void setLeftRigthPadding(){
         int paddingDp = 8;
         float density = getResources().getDisplayMetrics().density;