libandroidfw: Revert null check in ApplyStyle

The out parameter `out_indices` is expected to be non-null
and so the extra null check adds a cost to performance
and opens the door to misusing the API by not supplying
`out_indices`.

Test: make libandroidfw_tests
Change-Id: Ie66fd837c5e24ec2838156e7b67f54c15cd27933