Decoder: Fixed initialization of first_slice_in_pic

To handle some errors, first_slice_in_pic was being set to 2.
This is now cleaned up and first_slice_in_pic is set to 1 only once per pic.
This will ensure picture level initializations are done only once even in case
of error clips

Bug: 33717589
Bug: 33551775
Bug: 33716442
Bug: 33677995

Change-Id: If341436b3cbaa724017eedddd88c2e6fac36d8ba
