Bhv 10403 anishlg

updated with review comments.

1. the startYear or endYear bounds will get adjusted according to the given value.
2. the picker also will get updated with the new startYear and endYear bounds.
Let's wrap this block, `this.updateYearPicker()` and `this.$.yearPicker.render()` into a check to validate they're required before doing the work.

Also, in your for loop, you could do:
```
for(i=count; i <= yearDiff; i++) {
```
Which avoids the calculation (`yearDiff - count`) for every iteration.