MainActivity: inline nested function for dynamic menu

Make the UI more flexible and
slightly easier to understand.

app/src/main/java/it/danieleverducci/lunatracker/MainActivity.kt

@@ -480,6 +480,9 @@ class MainActivity : AppCompatActivity() {
 
         val pickedDateTimeBegin = dateTimePicker(event.time, datePickerBegin) { pickedTime: Long ->
             sleepStart = pickedTime
+            if (datePickerEnd.visibility == View.GONE) {
+                sleepEnd = pickedTime
+            }
             updateFields()
         }